Scintillator crystal body, method for manufacturing the same, and radiation detector

ABSTRACT

In a scintillator used for radiation detection, such as an X-ray CT scanner, a scintillation crystal body having a unidirectional phase separation structure is provided which has a light guide function for crosstalk prevention without using partitions. The phase separation structure includes a first crystal phase and a second crystal phase having a refractive index larger than that of the first crystal phase and which have a first principal surface and a second principal surface, these principal surfaces being not located on the same plane, the first principal surface and the second principal surface have portions to which the second crystal phase is exposed, and a portion of the second crystal phase exposed to the first principal surface and a portion of the second crystal phase exposed to the second principal surface are connected to each other.

TECHNICAL FIELD

The present invention relates to a scintillator crystal body, a methodfor manufacturing the same, and a radiation detector, and moreparticularly relates to a scintillator crystal body which emits light byradiation, a method for manufacturing the same, and a radiation detectorusing the above scintillator crystal body.

BACKGROUND ART

In an x-ray CT (computed tomography) apparatus used in medical sites andthe like, x rays passing through a photographic object are received byscintillators, and photodetectors detect light emitted therefrom. Inaddition, these detectors are disposed to form a two-dimensional array,and the scintillators are separated from each other by partitions so asnot to cause crosstalk of light.

Since having no contribution to the x-ray detection and degrading aspatial resolution, the partitions are each preferably formed as thin aspossible. For example, in PTL 1, a technique has been disclosed in whichafter many scintillator crystals are bonded to each other with anadhesive to form a scintillator array, the adhesive is removed byetching, and spaces formed thereby are filled with a titanium oxidepowder functioning as a partition material. In this case, according tothe above patent literature, the thickness of the partition can bedecreased to approximately 1 μm.

Since a related scintillator has no function to guide lighttherethrough, partitions functioning as a scattering surface and/or areflection surface are required. However, by the technique disclosed inPTL 1, although the thickness of the partition can be decreased, thepartition itself cannot be eliminated. In addition, in a manufacturingprocess, many steps, for example, from cutting of a scintillator tobonding for partition formation are required.

CITATION LIST Patent Literature

-   PTL 1 Japanese Patent Laid-Open No. 2008-145335

SUMMARY OF INVENTION

The present invention was made in consideration of the relatedtechniques described above and provides a scintillator crystal bodywhich is able to guide light and a method for manufacturing the same.

In addition, the present invention also provides a radiation detectorusing a scintillator crystal body which is able to guide light.

Solution to Problem

A scintillator crystal body which solves the problems described abovecomprises a phase separation structure which includes a first crystalphase and a second crystal phase having a refractive index larger thanthat of the first crystal phase and which has a first principal surfaceand a second principal surface, these two principal surfaces being notlocated on the same plane. In the scintillator crystal body describedabove, the first principal surface and the second principal surface haveportions at which the second crystal phase is exposed, and a portion ofthe second crystal phase exposed to the first principal surface and aportion of the second crystal phase exposed to the second principalsurface are connected to each other.

A method for manufacturing a scintillator crystal body which solves theabove problems has the steps of: mixing a material forming a firstcrystal phase and a material forming a second crystal phase; thenmelting the material forming a first crystal phase and the materialforming a second crystal phase; and then solidifying the materialforming a first crystal phase and the material forming a second crystalphase along one direction to form a eutectic compound.

A radiation detector which solves the problems described above has thescintillator crystal body described above and a photodetector, and thisscintillator crystal body is disposed so that the first principalsurface or the second principal surface faces the photodetector.

Advantageous Effects of Invention

According to the present invention, a scintillator crystal body which isable to guide light and a method for manufacturing the same can beprovided. In addition, the present invention can provide a radiationdetector using a scintillator crystal body which is able to guide light.

According to the present invention, since the scintillator crystal bodyitself has a function to guide light, a related manufacturing processincluding the steps from cutting of a scintillator to partitionformation is not required. In addition, a radiation detector having highuse efficiency of light can be provided only by disposing thescintillator crystal body on a photodetector array.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are each a perspective view showing one embodiment of ascintillator crystal body of the present invention.

FIGS. 2A and 2B are each a schematic view showing a method formanufacturing a scintillator crystal body of the present invention.

FIGS. 3A to 3F are each a scanning electron microscope image of ascintillator crystal body of the present invention.

FIG. 4A is a graph showing electron beam induced luminescence spectra ofa CsI—NaCl-system phase separation scintillator crystal body added withInI.

FIG. 4B includes scanning electron microscope images of aCsI—NaCl-system phase separation scintillator crystal body.

FIG. 4C is a graph showing electron beam induced luminescence spectra ofa CsI—NaCl-system phase separation scintillator crystal body added withTlI.

FIG. 5 is a graph showing Tl concentration dependence of luminousquantity and luminescence peak wavelength in a CsI—NaCl system.

FIG. 6 is an equilibrium diagram of a CsI—NaCl system.

FIGS. 7A and 7B are scanning electron microscope images showing thedifference of a phase separation structure by the composition of ascintillator crystal body.

FIG. 8 is a graph showing solidification rate dependence of thestructural period and diameter of a CsI—NaCl-system phase separationstructure.

FIG. 9A is a photograph showing a light guide property of a scintillatorcrystal body of Example 5 of the present invention.

FIG. 9B is a schematic view showing the light guide property of thescintillator crystal body of Example 5 of the present invention.

FIG. 9C is a transmission optical microscope image showing the lightguide property of the scintillator crystal body of Example 5 of thepresent invention.

FIGS. 10A to 10C are each a transmission microscope image showing thelight guide property of a scintillator crystal body in which one of RbI,CsBr, and RbBr is added to CsI of a second crystal phase.

FIG. 11 is a graph comparing the light guide property between a CsIneedle crystal and a CsI—NaCl crystal body.

FIGS. 12A to 12E are each a transmission optical microscope image of aphase separation scintillator crystal body.

FIGS. 13A to 13D are graphs each showing an excitation spectrum and aluminescence spectrum of a NaI:Tl-containing phase separationscintillator of the present invention.

FIG. 14 is a schematic equilibrium diagram of a NaI—RbI system.

FIGS. 15A to 15F are each a transmission optical microscope image of aphase separation scintillator crystal body.

FIGS. 16G to 16L are each a transmission optical microscope image of aphase separation scintillator crystal body.

FIGS. 17A to 17F are each a transmission optical microscope image of aphase separation scintillator crystal body.

FIGS. 18G to 18J are each a transmission optical microscope image of aphase separation scintillator crystal body.

FIG. 19 is a schematic equilibrium diagram of a CsBr—NaCl system.

FIGS. 20A to 20E are each a transmission optical microscope image of aphase separation scintillator crystal body.

FIG. 21 is a schematic view showing a radiation detector.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments for carrying out the present invention will bedescribed with reference to the drawings and the like. Although thereare various modes (various configurations and various materials) as theembodiments for carrying out the present invention, the common featuresin all the embodiments are that a scintillator crystal body has a phaseseparation structure which includes two phases, that is, a first crystalphase and a second crystal phase having a larger refractive index thanthat of the first crystal phase, and which has a first principal surfaceand a second principal surface, these two principal surfaces not beinglocated on the same plane; the first principal surface and the secondprincipal surface have portions to which the second crystal phase isexposed; and a portion of the second crystal phase exposed to the firstprincipal surface and a portion of the second crystal phase exposed tothe second principal surface are connected to each other. Accordingly,light inside the crystal phase having a high refractive index is totallyreflected by the crystal phase having a low refractive index locatedaround the high refractive-index crystal phase, and as a result, thelight travels in the high refractive-index crystal phase while beingguided. At this stage, since the crystal phase having a high refractiveindex is exposed to the first principal surface and the second principalsurface, and these exposed portions are connected to each other, thewaveguide (light guide) is performed toward the first principal surfaceor the second principal surface. In other words, this phenomenon may bedescribed such that light generated in the scintillator crystal bodytravels toward the first principal surface or the second principalsurface while being confined in the second crystal phase (that is, lightis prevented from spreading). As described above, in all the embodimentsof the present invention, the scintillator crystal body itself has awaveguide function (light guide function).

In addition, in each embodiment which will be described hereinafter, thestructure is preferably formed so that the first crystal phase which isa low refractive-index phase also has portions exposed to the firstprincipal surface and the second principal surface, and these exposedportions are connected to each other. By the structure described above,light in the second crystal phase can be more reliably guided (opticallyguided) to the first principal surface or the second principal surfacewithout being spread.

In addition, the structure is preferably formed so that the firstcrystal phase which is a low refractive-index phase is located in thesecond crystal phase which is a high refractive-index phase. By thestructure described above, a sufficient waveguide function (light guidefunction) can be obtained while the volume ratio of the first crystalphase in the scintillator crystal body is reduced.

Hereinafter, each embodiment will be described.

First Embodiment Columnar Crystal Structure in which a Primary Componentof a Second Crystal Phase is a Scintillator Material (CsI)

FIG. 1A is a perspective view showing a first embodiment of ascintillator crystal body of the present invention. In the scintillatorcrystal body having a phase separation structure which includes twophases according to this embodiment shown in FIG. 1A, a first principalsurface and a second principal surface have portions to which a secondcrystal phase is exposed, and a portion of the second crystal phaseexposed to the first principal surface and a portion of the secondcrystal phase exposed to the second principal surface are connected toeach other. In the embodiment shown in FIG. 1A, a first crystal phaseformed of many columnar crystals having a unidirectional characteristicis present in the second crystal phase. In particular, the phaseseparation structure includes two phases, that is, a first crystal phase11 and a second crystal phase 12 that is filled between side surfaces ofthe columnar crystals of the first crystal phase 11. Incidentally, thephase separation structure is a structure containing a plurality ofseparated phases which is obtained when a uniform state is changed. Thephase separation structure of this embodiment is a structure obtained insuch a way that when a uniform liquid state having no concrete structurein which constituent materials are melted is changed to a solid state,two crystal phases are simultaneously crystallized to have a certainperiodicity.

Besides a circle, an ellipse, and a quadrangle cross-sectional shape, across-sectional shape of a columnar crystal 18 forming the first crystalphase 11 may be formed of a plurality of crystal planes to have apolygonal shape. In addition, a diameter 13 of the columnar crystal 18is in a range of 50 nm to 30 μm and preferably in a range of 200 nm to10 μm. In addition, a period 14 of the columnar crystal 18 of the firstcrystal phase is in a range of 500 nm to 50 μm and preferably in a rangeof 1 to 20 μm. However, when a scintillator crystal body and aphotodetector or a photodetector array are used in combination,structural sizes thereof are preferably combined with each other so thatmany columnar crystals are arranged on a light receiving region of thephotodetector. For example, when the shape of the light receiving regionis a square having a 20-μm side, the columnar crystal preferably has astructural size in which the diameter is 5 μm and the period is 8 μm.Hence, in accordance with the size of the light receiving region, apartfrom the range of the structural size described above, columnar crystalshaving a small structural size are preferably used in combination. Inaddition, the range of the structural size is determined by theselection of material systems and the manufacturing conditions, and thetendency thereof will be described later.

Although depending on a manufacturing method, a thickness 15 of thescintillator crystal body can be adjusted to an arbitrary thickness.Since the scintillator crystal body detects radiation, the thickness ofthe crystal body must be sufficient to absorb its energy. For example,in the case of CsI which is a primary constituent material of thescintillator crystal body, a radiation length is 1.86 cm (distance atwhich the incident energy is reduced to 1/e) in a high energy region.Since it is calculated that 100% of energy is absorbed by 21 times theradiation length (39.06 cm), a sufficient thickness of the scintillatorcrystal body is 40 cm or less.

For example, in consideration of medical use in which a scintillatorcrystal body is frequently used in a low energy region of 1 MeV or less,the thickness 15 of the scintillator crystal body is in a range of 1 μmto 10 cm and preferably in a range of 10 μm to 10 mm. In addition, sincethe thickness varies depending on the set value of absorptioncoefficient of radiation, a scintillation crystal body having athickness out of the range described above may also be used in somecases.

Although preferably extending straight in a thickness direction 16, thecolumnar crystals may be interrupted, branched, or fused and may includea curved portion besides the straight portion, and in addition, thediameter portion of the columnar crystal may be partially changed. Whenthe direction of a solid-liquid interface in solidification isappropriately controlled, the columnar crystal can be curved.

The first crystal phase is preferably formed of a material containingNaBr (sodium bromide), NaCl (sodium chloride), NaF (sodium fluoride), orKCl (potassium chloride). In addition, NaCl is more preferable. Thecontent of NaBr, NaCl, NaF, or KCl contained in the first crystal phaseis 50 percent by mole or more and preferably in a range of 80 to 100percent by mole.

The second crystal phase preferably contains CsI (cesium iodide) as aprimary component. In this embodiment, the primary component of thesecond crystal phase indicates a material, the content of which is 50percent by mole or more, and this primary component is preferablycontained in an amount of 80 to 100 percent by mole in the secondcrystal phase.

When this primary component is CsI, as materials contained in the secondcrystal phase besides CsI, RbI (rubidium iodide), CsBr (cesium bromide),and RbBr (rubidium bromide) are preferable. More preferably, when CsIand RbI are contained in the second crystal phase, the ratio of RbI toCsI is preferably in a range of more than 0 to 20 percent by mole. Evenmore preferably, the ratio is 15 percent by mole or less. As in the casedescribed above, when CsBr is contained, the ratio of CsBr to CsI ispreferably in a range of more than 0 to less than 50 percent by mole.More preferably, the ratio is 20 percent by mole or less. As in the casedescribed above, when RbBr is contained, the ratio of RbBr to CsI ispreferably in a range of more than 0 to 10 percent by mole. In thiscase, although at most less than 50 percent by mole of CsBr may be addedto CsI as a primary component, when more than 20 percent by mole of RbIor more than 10 percent by mole of RbBr is added, the transmittance ofthe crystal body along the columnar crystal is remarkably decreased. Thereason for this is believed that a solid phase separation occurs, thatis, an RbI or an RbBr component which cannot be solid-solved in CsI inthe second crystal phase is precipitated.

In the selection of the above material systems, the importance of thescintillator crystal body of the present invention is the composition ofmaterials for the first and the second crystal phases. The compositionof materials forming the first and the second crystal phases containedin the scintillator crystal body of the present invention is preferablya composition at a eutectic point. The eutectic point is a point in anequilibrium diagram at which a eutectic reaction occurs and at which twotypes of solid solutions are simultaneously discharged from a liquidphase and solidification is completed.

In particular, the composition ratio of a preferable combination of thematerial system between the first and the second crystal phases of thisembodiment is shown in the following Table 1.

TABLE 1 EUTECTIC EUTECTIC FIRST CRYSTAL PHASE: COMPOSITION POINT SECONDCRYSTAL PHASE [mol %] [° C.] NaBr:CsI 40:60 432 NaCl:CsI 30:70 490NaF:CsI  5:95 599 KCl:CsI 40:60 447

Although equilibrium diagram data of the materials shown in Table 1 wasnot available, the equilibrium diagram thereof was developed throughintensive research carried out by the present inventors usingDifferential Thermal Analysis (DTA) and the like. In order to obtain anexcellent phase separation structure as shown in FIG. 1A, in general,the above compositions are preferably used. These compositionscorrespond to the respective eutectic points. However, the abovecomposition must not be strictly controlled, and an acceptable range of±4 percent by mole is preferably set for the composition. The acceptablerange is more preferably ±2 percent by mole. The reason the acceptablerange is limited to the vicinity of the composition as described aboveis that the phases have a eutectic relationship therebetween even in thestructure formation and that in the vicinity of the eutecticcomposition, an excellent structure as shown in FIG. 1A can be obtainedby unidirectional solidification. In other composition ranges, that is,when the composition is deviated by more than 2 percent by mole, onephase precipitates first, and in view of the structure formation, thestructure is disordered. However, since the eutectic composition ofTable 1 also includes a measurement error, although it is conceptionallyimportant that the acceptable range of the eutectic composition be ±2percent by mole, if an excellent structure is substantially obtained, adeviation of approximately ±4 percent by mole from the eutecticcomposition may also be accepted. In addition, since also including ameasurement error and the like, the eutectic point shown in Table 1indicates a temperature in the vicinity of the eutectic point and doesnot limit anything.

Next, the first and the second crystal phases may contain at least onecomponent other than those mentioned above, and in particular, thecomponent contained in the material forming the first crystal phase 11is preferably a component which is solid-solved in the first crystalphase 11 and which is not solid-solved in the second crystal phase 12.For example, NaBr may be added to NaCl, or KCl may be added to NaCl.Furthermore, the component contained in the material forming the secondcrystal phase 12 is preferably a component which is not solid-solved inthe first crystal phase 11 and which is solid-solved in the secondcrystal phase 12. For example, as described above, RbI may be added toCsI. In addition, when the primary component of the second crystal phase12 is CsI, and one of RbI, CsBr, and RbBr is added thereto to form thesecond crystal phase, a composition having a eutectic relationship withthe material composition forming the second crystal phase is preferablyused.

At least one material other than RbI, CsBr, and RbBr may be additionallyadded to the second crystal phase 12. In addition, as long as theformation of the phase separation structure is not disturbed, at leastone component which is solid-soluble in both phases may be added. Thepurpose of the addition of at least one material in an amount of 1percent by mole or more other than the addition of an ultra small amountof a luminescence center which will be described later is to control thelattice constant and/or the band gap and further is to control aluminescent color and the like.

In the phase separation structure of this embodiment, the second crystalphase 12 containing a scintillator material as a primary component isexcited by radiation irradiation and emits light. However, the case isnot limited to that described above, and in the present invention, atleast one of the first crystal phase and the second crystal phase mayemit light by radiation excitation, and it is more preferable when thetwo crystal phases emit light. Hence, when CsI is used as a primarycomponent of the second crystal phase, it is preferable that NaBr, NaCl,NaF, and KCl, each of which forms the first crystal phase 11, also emitlight although the radiation absorption ability thereof is inferior tothat of CsI. In particular, in order to increase the luminousefficiency, a small amount of a component functioning as theluminescence center is preferably added to base materials forming thefirst crystal phase 11 and the second crystal phase 12. However, whenonly reflection and refraction of light are taken into consideration, itis important that the second crystal phase which is at a highrefractive-index side at least emits light by radiation, and the reasonthe low refractive-index side is allowed to emit light is based on arelatively high probability of generation of effects, such asscattering, and is not based on the light guide property. In addition,the case described above in which the two crystal phases emit light doesnot exclude the case in which the second crystal phase absorbs lightemitted from the first crystal phase and the case opposite thereto.Furthermore, the case described above in which the two crystal phasesemit light also does not exclude the case in which before emittinglight, carriers generated in the first crystal phase enter the secondcrystal phase by diffusion and the case opposite thereto.

As the luminescence center, a large number of luminescence centers maybe selected in accordance with applications and the like, and a singleelement or a plurality of elements may be added. For example, Cu, Ag,Au, Ga, In, Tl, Sn, Pb, Sb, and Bi, each of which has an (ns)² electronconfiguration in an alkaline halide, and Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm, Yb, and Lu, each of which is a rare earth element, arepreferably selected. In addition, Na may also be selected. In view ofhigh luminance, as the luminescence center of the scintillator crystalbody, at least one of Tl, In, and Ga is more preferably contained. Whenthe above luminescence center is added, in accordance with applications,the element may be appropriately selected in consideration of desiredluminance, luminescence wavelength, luminescent decay time, and thelike. In addition, when added as described above, for example, theluminescence center is easily added to one of the two phases due to thephase separation structure, and concentration distribution may begenerated in some cases; however, no problems may arise thereby. Theluminescence by radiation described in the present invention alsoincludes, besides general scintillation (luminescence by radiationirradiation), photostimulated phosphorescence (trap sites of carriersgenerated by radiation irradiation are excited by light irradiation, andlight is emitted thereby).

As the important property of the phase separation scintillator crystalbody having a unidirectional characteristic of the present invention,the light guide may be mentioned. The refractive indices of the materialsystems forming the first crystal phase 11 and the second crystal phase12 are shown in Table 2.

TABLE 2 REFRACTIVE REFRACTIVE REFRACTIVE INDEX INDEX OF INDEX OF RATIOOF FIRST SECOND FIRST CRYSTAL MATERIAL CRYSTAL CRYSTAL PHASE/SECONDSYSTEM PHASE PHASE CRYSTAL PHASE NaBr:CsI 1.64 1.80 0.911 NaCl:CsI 1.551.80 0.861 NaF:CsI 1.32 1.80 0.733 KCl:CsI 1.49 1.80 0.828

For example, since the refractive index shown in Table 2 has wavelengthdependence and is changed by an additive, the value thereof is notstrict, and Table 2 is to show the difference in refractive index (shownby ratio in the table) between constituent materials. According toSnell's law, between materials having different refractive indices, iflight enters a low refractive-index medium at a certain angle from ahigh refractive-index medium, total reflection occurs, and at an anglesmaller than that, reflection and refraction occur. Hence, in the phaseseparation scintillator crystal body of the present invention, thegeneration of difference in the refractive index shown in Table 2indicates that light is not spread in the high refractive-index mediumby total reflection under some conditions. That is, since refraction andreflection are repeatedly performed, the high refractive-index medium ismore likely to relatively confine and transmit light. Hence, therefractive-index ratio (low refractive-index crystal phase/highrefractive-index crystal phase) is preferably at least less than 1. Inaddition, when only the total reflection condition is taken intoconsideration, it indicates that as the ratio of low refractiveindex/high refractive index is smaller, light is more difficult tospread. When only the refractive index is taken into consideration, asapparent from Table 2, the combination of NaF with CsI is mostpreferable, and KCl, NaCl, and NaBr are preferable in this order.

However, in this embodiment, CsI functioning as a high refractive-indexmedium forms the second crystal phase 12. That is, since the secondcrystal phase forms a matrix around the columnar crystals in thisembodiment, when the composition ratio (see Table 1) of the firstcrystal phase 11 forming the columnar crystals is low (for example, NaF:5 percent by mole), light tends to pass between sides of the columnarcrystals and to easily spread. In addition, when the composition ratioof the first crystal phase is high (for example, NaCl: 30 percent bymole), since the ratio of the second crystal phase formed of CsI havinga high x-ray response is decreased, the luminous quantity is liable torelatively decrease. Hence, in consideration of the combination betweenthe above two effects, among the above 4 types of material systems, NaClis more preferably used to form the first crystal phase 11 in view ofthe light guide. However, in consideration of the luminous efficiency byradiation excitation and the like, whether the combination is good orbad should be determined in accordance with each application; hence,whether the combination is good or bad is not simply determined by therefractive-index ratio and the composition ratio, and the above materialsystems are all important.

As described above, the phase separation scintillator crystal body ofthe present invention is characterized in that light is guided in adirection parallel to the height direction of the columnar crystal(direction between the first principal surface and the second principalsurface) and is not guided in a direction perpendicular thereto, forexample, by scattering and reflection. Accordingly, the crosstalk oflight can be suppressed without forming partitions in a single crystalgroup as in the related technique.

Second Embodiment Columnar or Lamella Structure in which a PrimaryComponent of a Second Crystal Phase is a Scintillator Material (NaI)

Next, a second embodiment will be described. In the second embodiment,NaI (sodium iodide) is used as a primary component of the second crystalphase, and a phase separation structure of the columnar crystalstructure shown in FIG. 1A or FIG. 1C or a lamella structure shown inFIG. 1B is obtained. Hereinafter, these structures will be described indetail.

FIGS. 1A, 1B, and 1C are each a perspective view showing one mode of ascintillator crystal body of this embodiment. The structure shown inFIG. 1A is similar to that in the above first embodiment, and hence adescription thereof will be partially omitted. In addition, thestructure as shown in FIG. 1A in which two phases, that is, a firstcrystal phase 11 formed of many columnar crystals having aunidirectional characteristic and a second crystal phase 12 which isfilled between side surfaces of the columnar crystals of the firstcrystal phase 11, are formed is hereinafter called a firstconfiguration. In addition, the structure as shown in FIG. 1B in which afirst crystal phase 11 and a second crystal phase 12 are each formed ofplate crystals standing in one direction and are alternately disposed inclose contact with each other is hereinafter called a secondconfiguration. Furthermore, the structure as shown in FIG. 1C in whichtwo phases, that is, a second crystal phase 12 formed of many columnarcrystals having a unidirectional characteristic and a first crystalphase 11 which is filled between side surfaces of the columnar crystalsof the second crystal phase 12, are formed is hereinafter called a thirdconfiguration.

A diameter 13 of the first crystal phase in the first configurationshown in FIG. 1A or a diameter 13 of the second crystal phase in thethird configuration shown in FIG. 1C and a period 14 in FIGS. 1A and 1Care preferably in the same ranges as those described in the firstembodiment. In addition, also in the second configuration shown in FIG.1B, although the crystal phase includes the plate crystals, when thediameter and the period are defined by a shorter-side side, a diameter13 and a structural period 14 of the plate crystals of the first crystalphase are preferably in ranges similar to those of the firstconfiguration.

In addition, when the scintillator crystal body of this embodiment isused in combination with a photodetector or a photodetector array, asdescribed in the first embodiment, structure sizes thereof arepreferably combined with each other so that a plurality of columnarcrystals or a plurality of plate crystals is arranged on a lightreceiving region of the photodetector. In the case of the plate crystalin the second configuration, the shorter-side side thereof may have asize similar to that of the columnar crystal. However, as for alonger-side side of the plate crystal, in accordance with theapplication, the size of a region surrounded by a first domain boundary17 shown in FIG. 1B and a second domain boundary 17 (not shown) ispreferably equal to or smaller than that of a light receiving region ofa detection pixel. The reason the size as described above is preferableis that light propagating along the longer-side side may not traveltoward a detector side and may spread in a lateral direction in somecases. However, when the light guide property at the shorter-side sideand that at the longer-side side are known in advance in associationwith the detector array, correction may be performed after an imagepick-up by the detector array, and hence, even when the size surroundedby the domain boundaries is larger than that of the detection pixel, theadvantages of the present invention may not always be obtained. Thescintillator crystal body may have a preferable light guide property atleast at the shorter-side side.

A thickness 15 of the scintillator crystal body is similar to that ofthe first embodiment.

Although this embodiment is realized when the second crystal phasecontains NaI as a primary component, CsI, RbI, NaCl or NaF is morepreferably contained as the first crystal phase. This indicates thatwhen a material forming the first crystal phase is a material systemwhich has a eutectic relationship with NaI, the structure of thisembodiment is preferably formed. One example of the availablerelationship between the material of the first crystal phase and that ofthe second crystal phase is as shown in Table 3. That is, in the case ofthe combination between NaI and CsI, the second configuration (FIG. 1B)is formed, the second crystal phase is NaI, and the first crystal phaseis CsI. In addition, in the case of the combination between NaI and RbI,the third configuration (FIG. 1C) is formed, the second crystal phase isNaI, and the first crystal phase is RbI. In the case of the combinationbetween NaI and NaCl, the first configuration (FIG. 1A) is formed, thefirst crystal phase is NaCl, and the second crystal phase is NaI.Furthermore, in the case of the combination between NaI and NaF, thesecond configuration (FIG. 1B) is formed, the second crystal phase isNaI, and the first crystal phase is NaF.

TABLE 3 FIRST SECOND COMBINATION CRYSTAL CRYSTAL OF MATERIALSCONFIGURATION PHASE PHASE CsI—NaI SECOND CsI NaI CONFIGURATION RbI—NaITHIRD RbI NaI CONFIGURATION NaCl—NaI FIRST NaCl NaI CONFIGURATIONNaF—NaI SECOND NaF NaI CONFIGURATION

Next, in the selection of the above material systems, the composition ofthe materials forming the first crystal phase and the second crystalphase is important in this embodiment.

In the four types of combinations of the material systems of thisembodiment shown in Table 3, preferable composition ratios are as shownin the following table 4, and the compositions are each preferably acomposition at a eutectic point.

TABLE 4 EUTECTIC EUTECTIC FIRST CRYSTAL PHASE: COMPOSITION POINT SECONDCRYSTAL PHASE [mol %] [° C.] CsI:NaI 51:49 428 RbI:NaI 50:50 505NaCl:NaI 40:60 573 NaF:NaI 18:80 596

As in the case of the above first embodiment, the acceptable range ofthe above composition is preferably ±4 percent by mole and is morepreferably ±2 percent by mole.

As described in the first embodiment, at least one material other thanthose described above may also be added to the first crystal phase andthe second crystal phase. For example, it is preferable that KI is addedto NaI, RbI is added to CsI, and CsI is added to RbI. As long as thestructure formation is not disturbed, at least one materialsolid-soluble in the two crystal phases may be added. In addition, thecontent of one of NaI, CsI, RbI, NaCl, and NaF, each of which is amaterial forming a corresponding crystal phase, is set to 50 percent bymole or more and is preferably set in a range of 80 to 100 percent bymole.

Since NaI which is a scintillator material is used for the secondcrystal phase of the phase separation structure in this embodiment, whenNaI is excited by radiation irradiation, light can be emitted. Althoughat least one crystal phase preferably emits light in this embodiment, itis more preferable when the two crystal phases both emit light. Inparticular, in order to increase the luminous efficiency, a small amountof at least one element functioning as the luminescence center(hereinafter simply referred to as “luminescence center”) is preferablyadded to base materials forming the first crystal phase 11 and thesecond crystal phase 12. As the luminescence center, elements similar tothose described in the first embodiment may also be used.

As the important property of the phase separation scintillator having aunidirectional characteristic of this embodiment, the light guide may bementioned. The refractive indices of the material systems forming thefirst crystal phase 11 and the second crystal phase 12 are shown inTable 5.

TABLE 5 REFRACTIVE REFRACTIVE INDEX OF INDEX OF RATIO FIRST SECOND OFLOW COMBINATION CRYSTAL CRYSTAL REFRACTIVE OF MATERIALS PHASE PHASEINDEX/HIGH CsI—NaI 1.80 1.85 0.973 RbI—NaI 1.61 1.85 0.870 NaCl—NaI 1.551.85 0.838 NaF—NaI 1.32 1.85 0.714

When only the total reflection condition is taken into consideration, asthe ratio of low refractive index/high refractive index is smaller,light is more difficult to spread. Hence, when the judgment is performedonly by the refractive index, it is apparent from this table that lightis most unlikely to spread in the NaF—NaI system, and light is unlikelyto spread in NaCl—NaI, RbI—NaI, and CsI—NaI systems in this order.

However, as described in the first embodiment, when the volume of thelow refractive-index material forming the first crystal phase is small,the visibility at the high refractive-index side becomes good, and lighttends to easily pass through and spread in a lateral direction. Althoughthe situation as described above may also occur in the NaCl—NaI (firstconfiguration) and the CsI—NaI (second configuration) systems, it doesnot indicate that the light guide effect is not obtained therein, andalthough light is sufficiently guided as compared to the case of a NaIsingle crystal, the effect is not so good as that of the other materialsystems of this embodiment. Although the NaF—NaI system belongs to thesecond configuration, since the form of NaF is frequently branched intothree ways, the visibility at the high refractive-index side is inferiorto that of the CsI—NaI system, and light is unlikely to pass through ina lateral direction. In the case in which NaI forms columnar crystals,that is, in the RbI—NaI system which is the third configuration, sincelight travels in the columnar crystal, in this configuration, light isdifficult to spread in a lateral direction. As described above, theability of light guide is not determined only by the ratio of therefractive index (difference in refractive index between a lowrefractive-index material and a high refractive-index material).

On the other hand, since light is emitted by radiation irradiation,although depending on the luminescence mechanism and/or the additivefunctioning as the luminescence center, radiation blocking capability ofCsI is most high and is followed by RbI, NaI, NaCl, and NaF in thisorder. As a result, when Tl is assumed as the luminescence center, CsI,RbI and NaI well emit light. Hence, as the material system for the phaseseparation scintillator, the CsI—NaI and the RbI—NaI systems are bothadvantageous since light is well emitted and a high signal strength canbe obtained. Under the condition described above, in general, theradiation blocking capability in consideration of the composition ratiois high in the order of CsI—NaI, RbI—NaI, NaF—NaI, and NaCl—NaI.However, since the radiation blocking capability has energy dependence,it may not be concluded in some cases that the radiation blockingcapability is high in the order of CsI, RbI, NaI, NaCl, and NaF.

Another significant property of this embodiment is a short luminescentdecay time of NaI:Tl and the like. In particular, NaI:Tl generally has adecay time (time to decay from the initial luminance to 1/e) ofapproximately 200 nsec (nano seconds) and is preferably used, forexample, for a CT apparatus which obtains many images at a high speed.As a comparative example, the luminescent decay time of CsI:Tl isapproximately 500 ns which is approximately 2.5 times that of NaI:Tl.The luminescent decay time of the material system of the presentinvention is shown in Table 6.

TABLE 6 MATERIAL SYSTEM LUMINESCENT DECAY TIME NaI—CsI 280 ns; 545 nsNaI—RbI 178 ns; 310 ns NaCl—NaI 135 ns; 273 ns NaI—NaF 153 ns; 429 ns

Since the material system of this embodiment is separated into twophases, the luminescent decay times corresponding to the two phases areobtained. This result is reasonably satisfactory since the fitting ofthe two components is well converged to the measurement data. Whenattention is paid to a long decay time of a material of each systemshown in Table 6 (for example, the decay time of CsI of the CsI—NaIsystem), it is found that the decay time is increased in the order ofNaCl—NaI, RbI—NaI, NaF—NaI, and CsI—NaI. Since the correspondencebetween the two phases and the two decay times in each system is notclear, FIG. 6 only shows that the two decay time constants are presentas an entire system.

Finally, the whole is summarized in Table 7.

TABLE 7 REFRACTIVE RADIATION BLOCKING LUMINESCENT INDEX RATIO CAPABILITYDECAY TIME MATERIAL (ORDER FROM (ORDER FROM (ORDER FROM SYSTEMCONFIGURATION SMALL VALUE) HIGH VALUE) SHORT VALUE) NaI—CsI (3) (4) (1)(4) NaI—RbI (1) (3) (2) (2) NaCl—NaI (2) (2) (4) (1) NaI—NaF (2) (1) (3)(3)

The number of each column in the table indicates the rank. The rank inthe column of configuration indicates the light guide performance, thatis, (1) the third configuration in which a high refractive-indexmaterial forms columnar crystals is ranked best; (2) the firstconfiguration in which a low refractive-index material forms columnarcrystals and (2) the specific second configuration (for example, beingbranched into three ways) are ranked at the same level; and (3) thesecond structure which is not specific is slightly inferior to the otherconfigurations.

When all the data are evaluated as a whole, it is found that the RbI—NaIsystem is averagely good in every column and that the NaF—NaI system isalso excellent in terms of the configuration and the refractive indexratio. As described above, it is preferable that in accordance with theperformance or the like which is thought important, a suitable system isdetermined and is selected for the use.

As described above, the phase separation scintillator crystal bodycontaining NaI of this embodiment has the properties in which light isguided along a direction parallel to the columnar crystals or the platecrystals and is not guided in a direction perpendicular thereto, forexample, by scattering and reflection. Hence, the crosstalk of light canbe suppressed without forming partitions in a single crystal group as inthe related technique.

Third Embodiment Columnar or Lamella Structure in which a PrimaryComponent of a Second Crystal Phase is not Limited to a ScintillatorMaterial

Next, a third embodiment will be described. The third embodiment is amode in which a primary component of a second crystal phase is notlimited to a scintillator material. In this embodiment, NaBr or NaCl isused as a material which is not a scintillator material. In addition, ascintillator material may also be used, and in this case, RbI, CsBr,RbBr, CsCl, or RbCl is used in this embodiment. A phase separationstructure of a columnar crystal structure shown in FIG. 1A or 1C or alamellar crystal structure shown in FIG. 1B is obtained using one of theabove materials as a primary component of the second crystal phase. Thisstructure will be described in detail. Since the configurations shown inFIGS. 1A, 1B, and 1C are already described in the above embodiments, inparticular, in the second embodiment, descriptions thereof will beomitted, and only the features of this embodiment will be described indetail. Although depending on a manufacturing method, a thickness 15 ofa scintillator crystal body can be adjusted to a desired thickness. Inorder to detect radiation, it is important that the thickness is largeenough to absorb the energy thereof. For example, as the radiationlength of the primary constituent material in the case of a high energyregion, RbI is 2.7 cm, CsBr is 2.1 cm, RbBr is 3.4 cm, CsCl is 2.4 cm,and RbCl is 4.6 cm. Since 21 times the radiation length of RbCl (96.6cm) is used as a rough indication to absorb 100% of energy, in the casein which the thickness is most increased, a thickness 15 of 97 cm orless is sufficient. Incidentally, the radiation length is an averagedistance necessary to decrease incident energy to 1/e.

In medical use in which a scintillator is frequently used in a lowenergy region of 1 MeV or less, for example, the thickness 15 is in arange of 1 μm to 15 cm and preferably in a range of 10 μm to 30 mm.Since being influenced by a set value of the absorption coefficient ofradiation, the thickness is not limited to the range described above.

This embodiment is realized when at least one phase uses as a primarycomponent one of RbI, CsBr, RbBr, CsCl, and RbCl, each of whichfunctions as a scintillator. As a more effective material system, one ofNaF, NaCl and NaBr is used in combination with one of RbI, CsBr, andRbBr. For example, a combination of NaCl with CsCl may be mentioned. Inaddition, a combination between RbCl and NaF or NaCl may also bementioned. The above combinations indicate that in view of the structureformation of the present invention, a material system having a eutecticrelationship can be selected. In addition, examples of availablerelationship between materials for the first crystal phase and those forthe second crystal phase are as shown in Table 8. In this embodiment,the second crystal phase, which is a high refractive-index layer, maycontain a material other than a scintillator material as a primarycomponent in some cases. For example, in the case of the combinationbetween RbI and NaBr, the second configuration (FIG. 1B) is formed, thesecond crystal phase, which is a high refractive-index phase, containsas a primary component NaBr which is not a scintillator material, andthe first crystal phase contains as a primary component RbI which is ascintillator material.

TABLE 8 FIRST SECOND CRYS- CRYS- COMBINATION TAL TAL OF MATERIALSCONFIGURATION PHASE PHASE NaF—RbI FIRST CONFIGURATION NaF RbI NaCl—RbIFIRST CONFIGURATION NaCl RbI RbI—NaBr SECOND CONFIGURATION RbI NaBrNaF—CsBr FIRST CONFIGURATION NaF CsBr NaCl—CsBr FIRST CONFIGURATION NaClCsBr NaBr—CsBr FIRST CONFIGURATION NaBr CsBr NaF—RbBr FIRSTCONFIGURATION NaF RbBr RbBr—NaCl SECOND CONFIGURATION RbBr NaClRbBr—NaBr THIRD CONFIGURATION RbBr NaBr NaCl—CsCl FIRST CONFIGURATIONNaCl CsCl NaF—RbCl FIRST CONFIGURATION NaF RbCl RbCl—NaCl FIRSTCONFIGURATION RbCl NaCl

Next, in the selection of the above material systems, the composition ofthe material forming the first crystal phase and that forming the secondcrystal phase is important in the present invention.

Among the 12 types of combinations of the representative materialsystems shown in Table 8, desirable composition ratios are as shown inthe following Table 9, and each combination is preferably composition ata eutectic point. The eutectic point is a point in an equilibriumdiagram at which a eutectic reaction occurs and at which at least twotypes of crystals are simultaneously crystallized from a liquid phaseand solidification is completed.

TABLE 9 EUTECTIC EUTECTIC FIRST CRYSTAL PHASE: COMPOSITION POINT SECONDCRYSTAL PHASE [mol %] [° C.] NaF:RbI  6:94 606 NaCl:RbI 35:65 482RbI:NaBr 53:47 448 NaF:CsBr  6:94 595 NaCl:CsBr 40:60 463 NaBr:CsBr41:59 465 NaF:RbBr 10:90 630 RbBr:NaCl 54:46 501 RbBr:NaBr 55:45 497NaCl:CsCl 35:65 486 NaF:RbCl 15:85 624 RbCl:NaCl 56:44 550

As in the case of the first embodiment, the acceptable range of theabove composition is preferably ±4 percent by mole and more preferably±2 percent by mole.

In addition, as described in the first embodiment, at least one materialother than those mentioned above may also be added to the first crystalphase and the second crystal phase. For example, NaBr may be added toNaCl, CsI or RbBr may be added to RbI, CsI, RbBr, or CsCl may be addedCsBr, and RbI or CsBr may be added to RbBr. More preferably, when CsI orRbBr are added to RbI of a primary component, the ratio of CsI to RbI isin a range of more than 0 to 20 percent by mole, and the ratio of RbBrto RbI is in a range of more than 0 to less than 50 percent by mole. Asin the described above, when CsI, RbBr, or CsCl are added to CsBr of aprimary component, the ratio of CsI to CsBr is in a range of more than 0to 50 percent by mole, and the ratio of RbBr to CsBr is in a range ofmore than 0 to 25 percent by mole, and the ratio of CsCl to CsBr is in arange of more than 0 to less than 50 percent by mole. As in thedescribed above, when RbI, or CsBr are added to RbBr of a primarycomponent, the ratio of RbI to RbBr is in a range of more than 0 to 50percent by mole, and the ratio of CsBr to RbBr is in a range of morethan 0 to 15 percent by mole. In this embodiment, since one of RbI,CsBr, RbBr, CsCl, and RbCl, each of which is a scintillator material, isalso used in the phase separation structure, when the above material isexcited by radiation irradiation, light can be emitted. Also in thisembodiment, although at least one of the crystal phases preferably emitslight, it is more preferable that the two crystal phases emit light. Inparticular, in order to increase the luminous efficiency, a small amountof at least one element functioning as the luminescence center(hereinafter simply referred to as “luminescence center”) is preferablyadded to base materials forming the first crystal phase 11 and thesecond crystal phase 12. As the luminescence center, elements similar tothose described in the first embodiment may also be used. As theimportant property of the phase separation scintillator having aunidirectional characteristic of the present invention, the light guidemay be mentioned. The refractive indices of the material systems formingthe first crystal phase 11 and the second crystal phase 12 are shown inTable 10.

TABLE 10 REFRACTIVE REFRACTIVE RATIO OF LOW INDEX OF INDEX OF REFRACTIVEFIRST SECOND INDEX/HIGH COMBINATION CRYSTAL CRYSTAL REFRACTIVE OFMATERIALS PHASE PHASE INDEX NaF—RbI 1.32 1.61 0.82  NaCl—RbI 1.55 1.610.963 RbI—NaBr 1.61 1.64 0.982 NaF—CsBr 1.32 1.71 0.772 NaCl—CsBr 1.551.71 0.906 NaBr—CsBr 1.64 1.71 0.959 NaF—RbBr 1.32 1.53 0.863 RbBr—NaCl1.53 1.55 0.987 RbBr—NaBr 1.53 1.64 0.933 NaCl—CsCl 1.55 1.65 0.939NaF—RbCl 1.32 1.53 0.863 RbCl—NaCl 1.53 1.55 0.987

When only the total reflection condition is taken into consideration, asthe ratio of low refractive index/high refractive index is smaller,light is more difficult to spread. Hence, when judgment is performedonly by the refractive index, in the systems shown in Table 10, it isfound that light is most unlikely to spread in CsBr—NaF. However, as forRbI—NaBr, RbBr—NaCl, RbBr—NaBr, and RbCl—NaCl shown in Table 10, an RbI,RbBr, and RbCl side functioning as a scintillator each form a lowrefractive index phase. Accordingly, since the phase emitting light byradiation excitation is different from the phase responsible for lightguide, the performance thereof is inferior to the other combinations;however, since effects, such as, scattering, may be generated at arelatively high probability, the above combination has an excellentlight guide function compared to a single-phase scintillator crystalbody.

In addition, when CsBr which has the highest refractive index in Table10 forms the second crystal phase of the first configuration, and as inthe combination with NaF, when the volume of a low refractive indexmaterial forming the first crystal phase is small (determined by thecomposition ratio and the density), light is liable to pass through in alateral direction and spread. In addition, in the case of the secondconfiguration, light is also liable to pass through in a lateraldirection and spread. This condition does not indicate that the effectsof this embodiment cannot be obtained but indicates that although lightis sufficiently guided as compared, for example, to the case of a CsBrsingle crystal, the performance is not so good as that of the othermaterial systems of this embodiment. On the contrary, the thirdconfiguration, that is, the RbBr—NaBr system, can be considered as aconfiguration in which light in not likely to spread in a lateraldirection. That is, since the high refractive-index second crystal phasecontaining NaBr as a primary component forms the columnar crystals, in aportion of light generated in the first crystal phase containing RbBr asa primary component, light incident on the second crystal phase formedof the columnar crystals is, for example, scattered, so that thetraveling path of the light is changed; hence, in some cases, the lightthus scattered is guided by a light guide function and is suppressedfrom spreading. As described above, the capability to guide light is notdetermined only by the ratio of the refractive index (difference inrefractive index between a low refractive-index material and a highrefractive-index material).

On the other hand, since light is emitted by radiation irradiation,although depending on the light emission mechanism and/or the additivefunctioning as the luminescence center, when x-ray energy is in a rangeof approximately 16 to 33 KeV, the linear attenuation coefficient ofeach of RbI, CsBr, RbBr, CsCl, and RbCl is approximately equal to thatof CsI. In addition, the linear attenuation coefficient of each of theabove materials except CsCl is higher than that of CsI, in order to emithigh luminescent light, the materials are preferably used in the energyrange described above.

As described above, the phase separation scintillator crystal body whichcontains RbI, CsBr, RbBr, CsCl, or RbCl as a primary component of thisembodiment has features in which light is guided in a direction parallelto the columnar crystal or the plate crystal and is not guided in adirection perpendicular thereto, for example, by scattering andreflection. Accordingly, the crosstalk of light can be suppressedwithout forming partitions in a single crystal group as in the relatedtechnique.

Fourth Embodiment Lamella Structure in which a Primary Component of aSecond Crystal Phase is Formed of a Scintillator Material (AlkalineEarth Halide)

Next, a fourth embodiment will be described. In the fourth embodiment,an alkaline earth halide is used as a primary component of a secondcrystal phase, and a phase separation structure of the lamella structureshown in FIG. 1B is obtained. This phase separation structure will bedescribed in detail. Since the structure shown in FIG. 1B is alreadydescribed in the above embodiments, the features of this embodiment willonly be described in detail.

A scintillator crystal body of this embodiment is a phase separationstructure which includes a unidirectional first crystal phase and secondcrystal phase, and the second crystal phase is a formed of a materialwhich contains as a primary component one of barium iodide (BaI₂),barium bromide (BaBr₂), barium chloride (BaCl₂), strontium bromide(SrBr₂), and strontium chloride (SrCl₂), each of which is alkaline earthhalide. In addition, the feature of the scintillator crystal body ischaracterized in that at least one phase of the above structure emitslight by radiation excitation.

As a preferable phase corresponding to the above second crystal phase,sodium iodide (NaI) is for barium iodide (BaI₂), sodium bromide (NaBr)is for barium bromide (BaBr₂), and sodium chloride (NaCl) is for bariumchloride (BaCl₂). In addition, sodium bromide (NaBr) is preferable forstrontium bromide (SrBr₂), and sodium chloride (NaCl) is preferable forstrontium chloride (SrCl₂). However, since a phase separation structureis probably formed when a material system has a eutectic relationshipwith the above alkaline earth halide, other combinations are notexcluded.

In addition, at least one of the first crystal phase and the secondcrystal phase preferably contains at least one of europium (Eu), cerium(Ce), thallium (Tl), indium (In), and gallium (Ga) as a luminescencecenter.

The composition of the first crystal phase and the second crystal phaseis preferably a composition at a eutectic point. Examples of availablerelationships between materials for the first crystal phase andmaterials for the second crystal phase are as shown in Table 11.

TABLE 11 FIRST SECOND COMBINATION CRYSTAL CRYSTAL OF MATERIALSCONFIGURATION PHASE PHASE NaI—BaI₂ SECOND NaI BaI₂ CONFIGURATIONNaBr—BaBr₂ SECOND NaBr BaBr₂ CONFIGURATION NaCl—BaCl₂ SECOND NaCl BaCl₂CONFIGURATION NaBr—SrBr₂ SECOND NaBr SrBr₂ CONFIGURATION NaCl—SrCl₂SECOND NaCl SrCl₂ CONFIGURATION

Next, in the selection of the above material systems, the composition ofthe material forming the first crystal phase and the material formingthe second crystal phase is important in this embodiment.

Among the five types of combinations of the material systems of thisembodiment shown in Table 11, preferable composition ratios are as shownin the following Table 12, and the compositions are each preferably acomposition at a eutectic point.

TABLE 12 EUTECTIC EUTECTIC FIRST CRYSTAL PHASE: COMPOSITION POINT SECONDCRYSTAL PHASE [mol %] [° C.] NaI—BaI₂ 50:50 491 NaBr—BaBr₂ 60:40 600NaCl—BaCl₂ 60:40 651 NaBr—SrBr₂ 40:60 457 NaCl—SrCl₂ 48:52 565

As in the first embodiment, the acceptable range of the abovecomposition is preferably ±4 percent by mole and more preferably ±2percent by mole. Since an alkaline earth halide which is a scintillatormaterial is used for the second crystal phase of the phase separationstructure of this embodiment, when being excited by radiationirradiation, the second crystal phase can be made to emit light.Although at least one of the crystal phases preferably emits light inthis embodiment, it is more preferable that the two crystal phases emitlight. In particular, in order to increase the luminous efficiency, asmall amount of an element functioning as the luminescence center ispreferably added to base materials forming the first crystal phase 11and the second crystal phase 12. As the luminescence center, at leastone of europium (Eu), cerium (Ce), thallium (Tl), indium (In), and thegallium (Ga) is preferably contained.

As the important property of the phase separation scintillator having aunidirectional characteristic of this embodiment, the optical guide maybe mentioned. The refractive indices of the material systems eachforming the first crystal phase 11 and the second crystal phase 12 areshown in Table 13.

TABLE 13 REFRACTIVE REFRACTIVE RATIO OF LOW INDEX OF INDEX OF REFRACTIVEFIRST SECOND INDEX/HIGH MATERIAL CRYSTAL CRYSTAL REFRACTIVE SYSTEM PHASEPHASE INDEX NaI—BaI₂ 1.85 (2.0)  0.925 NaBr—BaBr₂ 1.64 (1.8)  0.911NaCl—BaCl₂ 1.55 1.64 0.945 NaBr—SrBr₂ 1.64 (1.75) 0.937 NaCl—SrCl₂ 1.551.65 0.939

As described above, the phase separation scintillator crystal body whichcontains an alkaline earth halide of this embodiment has features inwhich light is guided in a direction parallel to the plate crystal andis not guided in a direction parallel thereto, for example, byscattering and reflection. Accordingly, the crosstalk of light can besuppressed without forming partitions in a single crystal group as inthe related technique.

Next, a method for manufacturing the scintillator crystal body of eachof the above embodiments will be described. A method for manufacturing ascintillator crystal body according to this embodiment includes thesteps of: mixing a material forming a first crystal phase and a materialforming a second crystal phase; then melting a mixture of the materialforming a first crystal phase and the material forming a second crystalphase; and then solidifying the material forming a first crystal phaseand the material forming a second crystal phase along one direction toform a eutectic compound.

The method for manufacturing a scintillator crystal body of thisembodiment can be realized by any method in which a desired materialsystem having an optimal composition is melted and solidified to have aunidirectional characteristic. In particular, since it is required tocontrol the temperature gradient to obtain a smooth solid-liquidinterface, the control is preferably performed so that the temperaturegradient at the solid-liquid interface of a mixture is 30° C./mm ormore. However, in order to avoid cracks and the like caused by a thermalstress applied to the crystal, the temperature gradient may be decreasedas long as the structure formation of each of the above embodiments isnot disturbed. In addition, in order to suppress the generation ofcracks and the like, reheating is preferably performed so as not toagain melt a portion which is already formed into the crystal body. Inaddition, as for the formable composition range of the eutecticcomposition, ±4 percent by mole and preferably ±2 percent by mole of theeutectic composition are described; however, the present inventorsbelieve that the crystal body of the present invention should be formedwithin a range so-called “Coupled Eutectic Zone” in which an intrinsicrelationship of material system holds among the acceptable range, thetemperature gradient, and the solidification rate.

FIGS. 2A and 2B are each a schematic view showing a method formanufacturing a scintillator crystal body of this embodiment.

As shown in FIGS. 2A and 2B, according to a Bridgman method, after asample sealed, for example, in a cylindrical quartz tube so as not to beoxidized is placed in a vertical direction, since a heater or the sampleis moved at a predetermined rate, the solidification interface of thesample can be controlled; hence, the phase separation scintillatorcrystal body of this embodiment can be manufactured.

In particular, an apparatus shown in FIG. 2A includes a heater portion21 having a length equivalent to that of a sample 23 and a water-coolingportion 22 which realizes a temperature gradient of 30° C./mm or more ata solid-liquid interface of the sample 23 which is a mixture substance.

In addition, as an apparatus shown in FIG. 2B, an apparatus in which twowater-cooling portions 22 are provided top and bottom and a heaterportion 21 corresponds only to a part of the sample 23 may also be used.Furthermore, a method performing steps equivalent to those describedabove may also be used. However, when the solid-liquid interface can bemade smooth, the temperature gradient may be less than 30° C./mm.

A method for pulling up a crystal from a melt, such as a Czochralskimethod, may also be used for the formation. In this case, sincesolidification is not performed in a sample container of a Bridgmanmethod, the solid-liquid interface can be formed without receiving anyinfluences of the container wall surface, and hence the above method ismore preferable. Furthermore, although a floating zone method may alsobe used for the formation, since the material system of this embodimenthas almost no infrared absorption properties, direct heating by infraredradiation cannot be performed as a heating method, and hence an additionmaterial and the like must be devised.

In particular, in a Bridgman method, although the solidification rate ofthe sample must be set so as to make the solid-liquid interface as flatas possible, heat exchange is primarily performed through the sidesurface of the sample, and hence the solidification rate depends on thediameter of the sample. That is, when the diameter of the sample islarge, the heat exchange takes a long time, and the solid-liquidinterface is considerably warped unless the solidification rate isdecreased, so that a curved columnar crystal may be formed. The reasonfor this is that the growth direction of the columnar crystal isapproximately vertical to the solid-liquid interface. Furthermore, whenthe solidification rate is faster with respect to the sample size, thesolid-liquid interface cannot be maintained flat and smooth, and microundulations are generated to cause the generation of dendrites; hence,it is important to avoid the above situation. Hence, a sufficienttemperature gradient at the solid-liquid interface of the sample is set,and at the same time, the solidification rate (pull-down rate of thesample by the apparatus) of the sample is preferably set to 850 mm/houror less. The solidification rate is more preferably set to 500 mm/houror less and is even more preferably set to 300 mm/hour or less.

In addition, regardless of the method, after the solid-liquid interfaceis controlled to have a convex surface from the solid side toward theliquid side so that the crystal body is formed into an optically singlegrain, and after a single grain is once formed, the convex surface ispreferably controlled to a flat surface. However, as long as nopractical problems may occur, a crystal formed with a convex-shapedsolid-liquid interface may also be processed by cutting for the use.

In addition, the diameter and the period of the columnar crystal of thephase separation scintillator crystal body depend on the solidificationrate of the sample, and in particular, it is believed that as for theperiod of the columnar crystals, the following formula holds. When theperiod is represented by λ and the solidification rate is represented byv, λ²×v=constant holds. Hence, if there is a desired structural period,the solidification rate is inevitably and roughly restricted by theabove formula. On the other hand, as the restriction of themanufacturing method, there is a solidification rate which can controlthe solid-liquid interface to be flat and smooth, and hence, the periodλ is set in a range of 500 nm to 50 μm. In addition, in accordance withthe above range, the diameter of the columnar crystal is also set in arange of 50 nm to 30 μm.

In this embodiment, the cross-sectional shape of the columnar crystalmay not be circular in some case, and when the above shape has anirregular form, the shortest diameter thereof is within the rangedescribed above. In addition, based on the average value of manycolumnar crystals, the ratio of the longest diameter to the shortestdiameter is preferably 10 or less. When the ratio is more than thatdescribed above, the structure should be appropriately regarded as alamella structure. However, although some columnar crystals among atremendous number of columnar crystals have a value of more than 10,when the average value is 10 or less, the columnar crystals are withinthe acceptable range. In addition, in view of the formation conditions,a lamellar structure can be more easily formed when the molar ratio ofthe material forming the first crystal phase and that forming the secondcrystal phase is closer to 1:1, and hence, in consideration of thistendency, the formation conditions and the additives are preferablyselected.

Next, a starting composition of a raw material forming the sample willbe described. Although the composition ratio of the material forming thefirst crystal phase and that forming the second crystal phase of thephase separation scintillator crystal body described above is the valueshown in each table, as for the starting composition, ±4 percent by moleor more may be deviated therefrom. That is, in a Bridgman method, whenthe entire sample is solidified in a unidirectional direction from thestate in which the entire sample is melted, a material deviated from theeutectic composition is first precipitated at an initial solidificationstage, and a remaining melt has the eutectic composition. In addition,in a Czochralski method, since a deviated portion is pulled up from themelt at an initial pull-up stage, it may also be preferable that afterpulling is once performed as a dummy operation so that the melt has theeutectic composition, pulling is again performed. An unnecessary portionmay be cut off after the crystal body is formed.

Next, the radiation detector of each of the above embodiments has theabove scintillator crystal body and a photodetector, and thescintillator crystal body is disposed so that the first principalsurface or the second principal surface faces the above photodetector.The scintillator crystal body is preferably disposed on thephotodetector with or without at least one protective layer interposedtherebetween.

When being combined with a photodetector, the phase separationscintillator crystal body of this embodiment can be used as a radiationdetector for medical use, industrial use, high-energy physical use, andspace use. In particular, since having a light guide function withoutproviding partitions and the like, the phase separation scintillatorcrystal body of this embodiment is preferably used when light must beguided in a specific direction toward a detector. In addition, the phaseseparation scintillator crystal body of this embodiment is effectivelyused in an x-ray CT apparatus in which partition formation is requiredand can replace a CsI needle crystal film of an x-ray flat paneldetector (FPD). In particular, since RbI, CsBr, RbBr, CsCl, or RbCl iscontained, when x-ray energy used for an image pick-up by mammography isapproximately 20 KeV, the phase separation scintillator crystal body ofthis embodiment preferably has a linear attenuation coefficientequivalent to that of CsI (cesium iodide). In the above uses, theluminescence wavelength of the scintillator may be adjusted by additionof another material or a luminescence center to base materials so as tofit with light-receiving sensitivity characteristics of a photodetector.

Furthermore, the detector and the phase separation scintillator crystalbody of this embodiment are preferably bonded or disposed to each otherwith or without at least one film or layer functioning as a protectivelayer, an antireflection layer, or the like.

Example 1

Examples 1 to 8 are examples corresponding to the first embodimentdescribed above. The examples will be sequentially described fromExample 1. In Example 1, powders obtained by mixing NaBr, NaCl, NaF, andKCl (each material forming a first crystal phase) in an amount,respectively, of 40, 30, 5, and 40 percent by mole with CsI (materialforming a second crystal phase composition) were prepared, and thepowders were separately vacuum-sealed in respective quartz tubes,thereby forming samples. Next, after each of those samples wasintroduced into a Bridgman furnace as shown in FIG. 2A and was heated to800° C. so as to be entirely melted (fused), the temperature was heldfor 30 minutes, and the melt temperature was then decreased to atemperature 20° C. higher than the eutectic point shown in Table 1.Subsequently, the sample was pull down at a rate of approximately 10mm/hour so that the sample was sequentially solidified from a lowerportion thereof.

In addition, when the sample entered a region in which cooling water ofthe furnace was circulated, the difference in temperature at asolid-liquid interface which was the boundary between a portion at whichthe sample was melted and a portion at which the sample was solidifiedwas set to 30° C./mm or more. As described above, the sample wassolidified along one direction, so that a eutectic compound wasgenerated.

The four types of samples thus formed were processed by cutting, and thestructures thereof were observed by a scanning electron microscope(SEM). As a result, as shown in FIGS. 3A and 3B, in the CsI—NaCl system,the structure (structure viewed from a first principal surface and asecond principal surface) of a plane perpendicular to the solidificationdirection was as shown in FIG. 3A, and the structure in a paralleldirection was as shown in FIG. 3B. In addition, by a compositionanalysis provided for the SEM, it was found that the columnar crystalcontained NaCl and the periphery thereof contained CsI. As describedabove, it was found that the structure was formed such that manycolumnar crystals of NaCl each had a unidirectional characteristic, andCsI surrounded the peripheries of the columnar crystals.

Although the structure viewed from the first principal surface and thatviewed from the second principal surface were described using only onedrawing, the reason for this is that since the structure viewed from thefirst principal surface and that viewed from the second principalsurface were very similar to each other, one of the structures wasrepresentatively shown in the figure, and it was also confirmed that thefirst crystal phase of columnar crystals and the second crystal phasehaving a high refractive index were both exposed to the first principalsurface and the second principal surface. In addition, as shown in FIG.3B, it was also confirmed that those exposed portions were connected toeach other.

In the other examples which will be described below, although thestructure viewed from the first principal surface and the structureviewed from the second principal surface will be described only usingsome of drawings; however, as described above, it should be understoodsince the structures of the two principal surfaces are very similar toeach other.

As in the case described above, in the CsI—NaF system, the structure(structure viewed from the first principal surface, the second principalsurface) of a plane perpendicular to the solidification direction was asshown in FIG. 3C and the structure in a parallel direction was as shownin FIG. 3D. In addition, it was found that the columnar crystalcontained NaF and the periphery thereof contained CsI.

As for the outstanding CsI—NaBr system and CsI-KCl system, it was foundthat when planes perpendicular to the solidification direction(structures viewed from the first principal surface, the secondprincipal surface) were as shown in FIGS. 3E and 3F, respectively, andthe columnar crystals contained NaBr and KCl, respectively. In thisexample, in the KCl system, in a part of the region of the image,columnar crystals were observed as if being connected to each other, forexample, by fluctuation of formation conditions; however, the essence ofthe present invention was not influenced thereby.

Accordingly, the phase separation scintillator structure of the presentinvention in which the second crystal phase included CsI was confirmed.

Example 2

The effect obtained by adding a luminescence center will be describedusing the CsI—NaCl system formed in Example 1. First, according to thesample formed in Example 1 to which the luminescence center was notadded, it was found that a luminescent color by x ray excitation(tungsten bulb; 60 kV; 1 mA) was pale, and light was emitted byradiation excitation without any addition. However, according to theproperties of the material system in this case, since the state wasformed in which a small amount of Na (CsI:Na was also known to emitlight although the luminescent mechanism was not understood) wasspontaneously added to CsI, luminescence caused thereby was alsoincluded; however, the luminescence center was not intentionally added.

Next, by a manufacturing method similar to that of Example 1, sampleswere formed by adding 0.01 percent by mole of InI (indium iodide (I)),0.01 percent by mole of TlI (thallium iodide (I)), and 0.01 percent bymole of Ga (added in a metal state) to the CsI—NaCl system. As in thecase described above, when each luminescence was confirmed by x-rayexcitation, as for the color touch of the luminescence by visualinspection, green was obtained by InI addition, and white was obtainedby TlI addition and Ga addition. All of samples had very high luminance.As described above, the luminescence was also confirmed by addition ofthe luminescence center of In, Tl, or Ga. In this case, the additioncomposition of the luminescence center is not limited to 0.01 percent bymole.

Hence, in the sample added with InI, in order to investigate where andhow this phase separation scintillator crystal body emits light, acathode luminescence (CL) which could measure local luminescence byelectron beam excitation was used. Focused electron beams of 5 keV wereused, and a luminescence spectrum of the columnar crystal containingNaCl and that of the region containing CsI located therearound wereseparately measured. At this time, the measurement was very carefullyperformed so as to prevent electron beams of an excitation source fromwidely spreading in the crystal body and exciting the other portion. Inaddition, although the difference in spectrum between x-ray excitationand electron beam excitation may arise to a certain extent, since theenergy of electron beams is high such as 5 keV, it is believed that basematerial excitation similar to the x-ray excitation mainly occurs ratherthan direct excitation of the luminescence center, and hence theluminescence properties were evaluated by this method.

The results are shown in FIG. 4. Two spectra of FIG. 4A are each theaverage of spectra obtained by electron beam excitation of six points ineach of two scanning electron microscope images shown in FIG. 4B. Inaddition, three points out of the six excited points are illustrated ineach image shown in FIG. 4B. First, a solid line spectrum indicates theluminescence in the region containing CsI, and green luminescence wasobserved. In addition, as in the case described above, a dotted linespectrum was obtained when the columnar crystal containing NaCl wasexcited, and it was confirmed that a luminescence peak position wasalmost the same as that of the solid line spectrum and that theluminescence was green. As described above, it was shown that the phaseseparation scintillator crystal body of the present invention emittedlight regardless of whether the luminescence center was added or not andat the same, as one example, the first and the second crystal phasesboth emitted light in the case of the In center.

As in the case described above, measurement was performed on the samplein which TlI was added to CsI—NaCl, and the spectra are shown in FIG.4C. As in the case of the InI addition, it was confirmed that crystalphases both emitted light when being excited.

When the above NaCl phase was excited, in both InI addition and TlIaddition, the intensity of the characteristic luminescence of In-addedNaCl at approximately 410 nm was small as compared to that of theluminescence at 530 nm, and the characteristic luminescence of Tl-addedNaCl at approximately 350 nm itself was not observed. However, sincecertain light could be extracted outside by the NaCl phase excitation,in the present invention, it was expressed that the two phases bothemitted light.

Example 3

This example relates to the luminous quantity with respect to a Tlconcentration in the system in which TlI was added to CsI—NaCl which wasa phase separation scintillator crystal body.

Light emitted by irradiation of x rays obtained at 60 kV and 1 mAwithout an Al filter using a tungsten bulb was integrated by anintegrating sphere, and based on the value thus obtained, the luminousquantity was relatively compared.

In FIG. 5, the luminescence wavelength and the relative luminance withrespect to the TlI concentration added to CsI—NaCl are shown. As aresult, it was found that the optimum value of the Tl concentration withrespect to the luminous quantity was in a range of 0.04 to 1.0 percentby mole. However, since the luminescence wavelength shifts to a longwavelength side in proportion to the concentration, the optimalconcentration may not be limited only to the above range inconsideration of a sensitivity curve of a light receiving element, andthe optimal range may be determined in consideration of the above twofactors.

Example 4

As the composition of the phase separation scintillator crystal body ofthe present invention, three samples shown by arrows in a phase diagramshown in FIG. 6, that is, CsI—NaCl (20 percent by mole), CsI—NaCl (28percent by mole), and CsI—NaCl (30 percent by mole), were formed by anapparatus similar to that of Example 1 shown in FIG. 2A and were formedby an apparatus as shown in FIG. 2B in which a heater portion wasnarrow, and a sample was locally melted, thereby forming totally 6 typesof samples.

In the case similar to that Example 1 in which solidification wasstarted after the entire sample was melted at an early stage, it wasfound that by all the samples formed as described above, as thestructure of the first principal surface and that of the secondprincipal surface, an excellent structure as shown in a scanningelectron microscope (SEM) image of FIG. 7A was obtained. However, in thecase of NaCl (20 percent by mole), a crystal was formed together withprecipitation of CsI dendrites in a solidification initial area of thesample as shown in FIG. 7B, and thereafter, an excellent region as shownin FIG. 7A was formed. In the cases of the other NaCl (28 percent bymole) and NaCl (30 percent by mole), a solidification initial area ofthe sample similar to that at a concentration of 20 percent by mole wasnot clearly formed, and a significant difference was not seen betweenthe above two samples. However, since the difference caused by thedifference in concentration of 2 percent by mole may also be partiallyinfluenced, for example, by impurities of raw materials, the presentinventors do not conclude that no difference was present.

As described above, when the entire sample was melted, since a materialdeviated from a eutectic composition was precipitated at a hightemperature in the solidification initial area of the sample, theremaining melt portion had the eutectic composition, and hence it wasbelieved that after the above precipitation, an excellent structurecould be formed.

In addition, in the case in which solidification was performed bymelting a local area of the sample as shown in FIG. 2B, in the system ofNaCl (20 percent by mole), CsI dendrites were present at any place ofthe sample as shown in FIG. 7B, and although some columnar crystals ofNaCl were formed therebetween, this structure was not a targetedstructure. However, in the case of NaCl (28 percent by mole) and NaCl(30 percent by mole), as shown in FIG. 7A, the columnar crystals of NaClwere formed over the entire area, and an excellent sample could beobtained.

However, it was observed that although being partially influenced byimpurities and the like, the structure of the sample of NaCl (28 percentby mole) was more distorted than that of the sample of NaCl (30 percentby mole). Hence, when the solidification was performed by local melting,it was found that since this solidification was not subjected to aprocess in which the composition was optimized in the sample when theentire was melted, the composition and the structure sensitivelyinfluenced each other.

Hence, in the phase separation scintillator crystal body of the presentinvention, although a slight fluctuation of the composition ofapproximately 2 percent by mole had not considerable adverse influenceon the structure formation, a considerable fluctuation of 10 percent bymole had adverse influences; hence, it was clear that the optimalcomposition was in the vicinity of the eutectic composition. Inaddition, it was found that when solidification was controlled after theentire sample was melted in the manufacturing, even if the compositionwas deviated, a deviated material was preferentially precipitated in thesolidification initial area of the sample, and an excellent region couldbe obtained from the remaining melt of the eutectic composition, andthis finding is important.

Example 5

The controllability of the structure size will be described using aCsI—NaCl system as an example. Four powders in which 30 percent by moleof NaCl was mixed with CsI were prepared in quartz seal tubes, and thesamples were formed by a manufacturing method similar to that ofExample 1. The pull-down rates of the samples were 10.4, 31.3, 94.0, and232 mm/hour.

The formed sample was cut along a plane perpendicular to thesolidification direction, and the surfaces of the sample (the firstprincipal surface, the second principal surface) were observed by a SEM,and the diameter and the period of the NaCl columnar crystal of thephase separation structure were obtained.

As a result, as shown in the graph of FIG. 8, the dependence wasobtained in which as the pull-down rate, that is, the solidificationrate, was decreased, the period and the diameter were increased. As forthe period, the formula of λ(period)=0.0897+11.7 (v^(−1/2)) wasapproximately satisfied. In this embodiment, v indicates the pull-downrate (solidification rate).

Hence, by the conversion, a structural period of 50 μm is obtained at apull-down rate of 0.055 mm/hour (1.319 mm/day)) close to 1 mm/day whichis a rough indication of a very low rate. By the conversion, astructural period of 500 nm is obtained at a pull-down rate of 813mm/hour close to 850 mm/hour which is a rough indication of an upperlimit of a fast rate. In addition, as for the diameter, the formula ofλ(diameter)=0.0412+4.78 (v^(−1/2)) was obtained. Hence, as in the caseof the period, by the conversion, a diameter of approximately 20.4 μm isobtained at a rate of 0.055 mm/hour, and a diameter of approximately 209nm is obtained at a rate of 813 mm/hour.

Hence, it was found that structure size could be widely controlled bythe control of the solidification rate. However, this example is justone example, and although the correlation of λ²×v=constant (v is thesolidification rate) also holds for other material systems, the constantis different.

Example 6

This embodiment relates to the light guide of a phase separationscintillator crystal body, and FIG. 9A shows the case in which a phaseseparation scintillator crystal body of In-doped CsI—NaCl (30 percent bymole) formed by a Bridgman method was disposed on the plane. Thethickness of the crystal was approximately 4 mm. At a crystal centralportion, characters printed on the plane were observed as if floatingthrough the crystal body, and this indicated that no light scatteringfactors were present in a direction perpendicular to the plane on whichthe characters were printed. In addition, in the periphery of thecrystal body, the characters printed on the plane could not be observed,and white color was only observed; hence, this indicated that light wasscattered in a direction perpendicular to the plane. This situation canbe described as follows. That is, as schematically shown in FIG. 9B, atthe central portion of the crystal, the NaCl columnar crystals whichwere the first crystal phase were aligned in a direction perpendicularto the plane on which the characters were printed, and at the peripheralportion, by the reason of the manufacturing method in which thesolidification interface was warped at the outside, the NaCl columnarcrystals were curved from the side surface to the upper surface. First,in the portion in which the columnar crystals aligned in the directionperpendicular to the plane on which the characters were printed, sinceno scattering factors (change in refractive index, structuralnon-uniformity, and the like in the direction perpendicular to the planeon which the characters were printed) were present, the characters onthe plane were observed as if floating, and hence this indicated thatlight was guided through a thickness of 4 mm.

Furthermore, since the columnar crystals were curved at the peripheralportion, no columnar crystals extending from the plane to the uppersurface were present, light was scattered thereby, and only white colorwas observed; hence, it was apparent that light was not guided towardthe upper surface. That is, this indicated that the light guide wasperformed in the direction along the columnar crystal, and that thelight guide was not performed in the direction perpendicular to that inwhich the columnar crystal extended. In addition, it was confirmed thatwhen an object was placed at the side surface of the crystal, the lightwas curved and guided to the upper surface thereof, and hence, even ifthe columnar crystal was curved, light was guided therealong.

Subsequently, when the portion in which the characters were observedthrough the crystal as shown in FIG. 9A was enlarged by an opticalmicroscope, and a transmission image was observed, an image shown inFIG. 9C was obtained (that is, image of the first principal surface, thesecond principal surface). In the transmission image, the NaCl columnarcrystals were clearly observed as black dots, and CsI which was thematrix side was observed brighter than that. That is, the situation inwhich a large transmission quantity of incident light was guided by theCsI matrix side was also exactly observed.

Hence, it was confirmed that the phase separation scintillator crystalbody of the present invention had properties of reliably guiding lightonly in the columnar crystal direction.

Example 7

This example relates to the case in which the second crystal phasecontains CsI as a primary component, and one of RbI, CsBr, and RbBr isadded thereto.

In this example, by using a CsI—NaCl system, samples in which thecompositions of RbI with respect to CsI were 15, 30, and 50 percent bymole, samples in which the compositions of CsBr were 20 and 50 percentby mole, and samples in which the compositions of RbBr were 10, 15, and50 percent by mole were formed by a manufacturing method similar to thatof Example 1.

Those samples were cut into pieces having a thickness of approximately200 μm along a plane perpendicular to the NaCl columnar crystal of thesample, and an optical microscope image was obtained by a transmissionarrangement for the first principal surface or the second principalsurface. The results are as shown in the following Table 14. In thetable, the samples are categorized by the light guide property obtainedwhen x percent by mole of a material A is added to CsI. X [percent bymole] in the table indicates the ratio of the material A in the secondcrystal phase, and the total of CsI and the material A is set to 100%.In addition, the light guide characteristics are shown by the following4 classes, that is,

: excellent light guide, ◯: slight degradation in light guide; Δ: lightguide property is inferior but light guide is performed along thestructure; x: no light guide by the structure.

TABLE 14 A X [mol %] LIGHT GUIDE PROPERTY RbI 15

30 Δ 50 X CsBr 20

50 ◯ RbBr 10 ◯ 15 Δ 50 X

From the above results, it was found that the addition range of RbI tothe primary component CsI was reasonably set in a range of more than 0to 20 percent by mole. Furthermore, it was found that the range of CsBrwas reasonably set in a range of more than 0 to less than 50 percent bymole, and the range of RbBr was reasonably set in a range of more than 0to 10 percent by mole.

In FIGS. 10A to 10C, transmission microscope photographs of the first orthe second principal surface of the sample having an excellent lightguide property are shown. FIG. 10A shows a sample in which 15 percent bymole of RbI was added, FIG. 10B shows a sample in which 20 percent bymole of CsBr was added, and FIG. 10C shows a sample in which 10 percentby mole of RbBr was added. In addition, although scratches and/ordefects shown in the figures were generated when the sample wasprocessed by cutting, the essence of the present invention was notinfluenced thereby. As described above, it was confirmed that even ifCsI was used as a primary component of the second crystal phase, and oneof RbI, CsBr, and RbBr was added thereto, the phase separationscintillator crystal body of the present invention could be formed.

Example 8

This example relates to radiation detection using the phase separationscintillator described in one of the above examples.

A phase separation scintillator crystal body cut into a thickness of 1mm was arranged on a photodetector array so that the first principalsurface or the second principal surface faced photodetectors, and as aresult, a radiation detector shown in FIG. 21 was formed. Referencenumeral 1 in the figure indicates the phase separation scintillatorcrystal body described in one of the above examples, reference numeral 2indicates the photodetector, and reference numeral 3 indicates asubstrate. When a single crystal having no partitions was irradiatedwith x rays, light diffused and propagated in the crystal; however, whenthe phase separation scintillator crystal body of this radiationdetector was irradiated with x rays, it was confirmed by an output ofthe detector array that light was suppressed from spreading.

Furthermore, it was confirmed that when the phase separationscintillator crystal body and the photodetector array were bonded toeach other so as not to form a space therebetween by using a resin, theoutput of the detector array was increased, and this indicated that alayer structure in consideration of extraction of light from the crystalbody to the detector portion was formed.

In addition, a Tl-added CsI needle crystal film having a thickness of430 μm was prepared by a common deposition method as a comparativeexample, and the spread of light was compared with a Tl-added CsI—NaClsystem having a thickness of 1.42 mm as the phase separationscintillator crystal body of the present invention. The sample wasirradiated with x rays obtained at 60 kV and 1 mA without an Al filterusing a tungsten bulb through an opening having a diameter of 100 μmprovided in a tungsten sheet having a thickness of 2 mm, and the lightintensity distribution at the bottom surface of the sample was measured.Measurement was performed using CCDs at a 50-micrometer pitch. Theintensity profile of the cross-section which passes through the peakvalue of the distribution is shown in FIG. 11. In FIG. 11, each profileis normalized by the peak value and the relative position is determinedwith respect to the peak position. Although the full width at halfmaximum (FWHM) of the CsI needle crystal film was approximately 340 μm,the FWHM of the CsI—NaCl crystal body of the present invention wasapproximately 160 μm, and it was found that the spread of light wassuppressed to not more than one half. Accordingly, although the crystalbody of the present invention had a thickness of no less than 1.42 mm,it was confirmed that the spread of light was suppressed as compared tothe CsI needle crystal film having a thickness of 430 μm which had beenthought to have a light guide effect, and that the crystal body of thepresent invention had advantages as a scintillator having a light guidefunction.

Example 9

Examples 9 to 11 are examples corresponding to the above secondembodiment. In this Example 9, NaI was used as a primary component ofthe second crystal phase. In particular, first, powders formed by mixingeach of CsI, RbI, NaCl, and NaF with NaI to have a concentration of 51,50, 40, and 18 percent by mole were prepared, and the powders wereseparately vacuum-sealed in respective quartz tubes, thereby formingsamples. Next, after the sample was placed in a Bridgman furnace asshown by the perspective view of FIG. 2A and was increased intemperature to 800° C. so as to be entirely melted, the temperature washeld for 30 minutes, and the melt temperature was then decreased to atemperature 20° C. higher than the eutectic point shown in Table 4.Subsequently, the sample was pulled down at a rate of approximately 10mm/hour so that the sample was sequentially solidified from a lowerportion thereof. In addition, when the sample entered a region in whichcooling water of the furnace was circulated, the difference intemperature at a solid-liquid interface which was the boundary between aportion at which the sample was melted and a portion at which the samplewas solidified was set to 30° C./mm or more. The four types of samplesthus formed were processed by cutting, and the transmission image of thefirst principal surface or the second principal surface was obtained byan optical microscope. The results are shown in FIGS. 12A to 12E.

FIG. 12A shows a plane (the first principal surface, the secondprincipal surface) perpendicular to the solidification direction of aNaI—CsI system. From FIG. 12A, the structure in which plate crystalswere alternately disposed in close contact with each other, that is, thesecond configuration, was confirmed. FIG. 12B shows a plane (the firstprincipal surface, the second principal surface) perpendicular to thesolidification direction of a NaI—RbI system. From FIG. 12B, thestructure in which many NaI columnar crystals were surrounded by RbI,that is, the third configuration, was confirmed. FIG. 12C shows a planeparallel to the solidification direction of a NaI—RbI system. From FIG.12C, it was confirmed that long NaI columnar crystals were grown alongthe solidification direction. That is, this indicated that aunidirectional phase separation structure was formed.

FIG. 12D shows a plane (the first principal surface, the secondprincipal surface) perpendicular to the solidification direction of aNaI—NaCl system.

FIG. 12E shows a plane (the first principal surface, the secondprincipal surface) perpendicular to the solidification direction of aNaI—NaF system. From FIG. 12E, it was found that the NaF plate crystalhad a three-branch structure.

In this example, since scratches and/or distortions of the image in someregions were caused by marks formed by polishing in the sampleformation, and blurs were caused by deliquescence of materials anddeviation of focus due to an inclined sample, the essence of the presentinvention was not influenced thereby.

The optical microscope images shown in FIGS. 12A to 12E were obtained bya transmission arrangement, and the NaI plate crystal side of theNaI—CsI system, the NaI columnar crystal side of the NaI—RbI system, theNaI matrix side of the NaI—NaCl system, and the NaI matrix side of theNaI—NaF system were more brightly observed. These results also indicatedthat the systems described above had a light guide function.

As described above, it was confirmed that the structure of the presentinvention in which one of the two phases was formed from NaI functionedas a phase separation scintillator and also had a light guide property.

Example 10

This example relates to the addition of a luminescence center.

By a manufacturing method similar to that of Example 9, samples of fourmaterial systems, that is, a NaI—CsI, a NaI—RbI, a NaCl—NaI, and aNaI—NaF system, each added with 0.01 percent by mole of TlI (thalliumiodide), were formed. When the luminescence thereof was confirmed byx-ray excitation, the luminescent color of NaI—CsI was pale, and theluminescent colors of the others were all blue. All of samples had veryhigh luminance. The excitation spectrum and the emission spectrum byultraviolet excitation of each sample are shown in FIGS. 13A to 13D. Thehorizontal axis indicates the wavelength, the spectrum at a shortwavelength side is an excitation spectrum, and the spectrum at a longwavelength side is an emission spectrum. The excitation spectrum wasmeasured at the peak position of the emission spectrum. The NaI—CsIsystem, the NaI—RbI system, the NaCl—NaI system, and the NaI—NaF systemcorrespond to FIGS. 13A, 13B, 13C, and 13D, respectively. From thesespectra, it was confirmed that the samples were all based on blue highluminescence of NaI:Tl known as a material system having high luminance.As described above, the phase separation scintillator crystal body ofthe present invention emits light by radiation excitation.

Example 11

This example relates to the composition of a phase separationscintillator.

From three types of samples of NaI—RbI (40, 48, and 50 percent by mole)indicated by arrows in an equilibrium diagram of a NaI—RbI system shownin FIG. 14, three types of phase separation scintillators were formedusing an apparatus similar to that of Example 9 shown in FIG. 2A. Inaddition, from these three types of samples, three types of phaseseparation scintillators were formed using the apparatus shown in FIG.2B in which the heater portion was narrow and the sample was locallyheated. As described above, totally six types of phase separationscintillators were prepared. The NaI—RbI (40 percent by mole) indicatedthat the molar ratio of NaI to RbI was 40:60. As in the case describedabove, in NaI—RbI (48 percent by mole), the molar ratio of NaI to RbIwas 48:52, and in NaI—RbI (50 percent by mole), the molar ratio of NaIto RbI was 50:50. In the case in which solidification was started afterthe entire sample was melted at an early stage as in the case of Example9, from all the samples, scintillators each having an excellentstructure as shown in FIG. 12B or 12C were obtained. However, in thecase of NaI—RbI (40 percent by mole), a crystal was formed together withprecipitation of NaI dendrites in a solidification initial area of thesample, and thereafter, a region of an excellent unidirectional phaseseparation structure was formed. As for NaI—RbI (48 percent by mole) andNaI—RbI (50 percent by mole), a dendrite region as that of NaI—RbI (40percent by mole) was not clearly formed at a solidification initialstage, and a significant difference was not seen between these twosamples. However, since the difference caused by the difference inconcentration of 2 percent by mole may also be partially influenced byimpurities of raw materials, it did not indicate that no difference waspresent. As described above, when the entire sample was melted, since amaterial deviated from a eutectic composition was precipitated in thesolidification initial area of the sample at a higher temperature thanthe eutectic point. Hence, it is believed that a remaining melt portionwas converged to the eutectic composition, and subsequently, preferablestructure formation was performed.

In addition, when solidification was performed while a local area of thesample was being melted as shown in FIG. 2B, NaI dendrites were presentat any place of the sample of the NaI—RbI (40 percent by mole) system,and a targeted structure was not obtained. However, in the case ofNaI—RbI (48 percent by mole) and NaI—RbI (50 percent by mole), thecolumnar crystals of NaI were formed over the entire area and anexcellent sample could be obtained. However, it was observed thatalthough being influenced by impurities and the like, the structure ofthe sample of NaI—RbI (48 percent by mole) was more distorted than thatof the sample of NaI—RbI (50 percent by mole). Hence, when thesolidification was performed by local melting, it was found that sincethis solidification was not subjected to a process in which thecomposition was optimized in the sample when the entire was melted, thecomposition and the structure sensitively influenced each other.

Hence, in the phase separation scintillator crystal body of the presentinvention, although a slight fluctuation of the composition ofapproximately 2 percent by mole had not considerable adverse influenceon the structure formation, a considerable fluctuation of 10 percent bymole had adverse influences; hence, it was clear that the optimalcomposition was in the vicinity of the eutectic composition. Inaddition, it was found that when solidification was controlled after theentire sample was melted in the manufacturing, even if the compositionwas deviated from the eutectic composition, a deviated material waspreferentially precipitated in the solidification initial area of thesample, and an excellent region could be obtained from the remainingmelt of the eutectic composition, and this finding is important.

Example 12

Examples 12 to 15 are examples corresponding to the above thirdembodiment and will be sequentially described.

In Example 12, powders each having a eutectic composition of the samematerial system as that shown in Table 9 were prepared and werevacuum-sealed in respective quartz tubes to form samples. Next, afterthe sample was placed in a Bridgman furnace as shown by the perspectiveview of FIG. 2A and was increased in temperature to 800° C. so as to beentirely melted, the temperature was held for 30 minutes, and the melttemperature was then decreased to a temperature 20° C. higher than theeutectic point shown in Table 9. Subsequently, the sample was pulleddown at a rate of approximately 10 mm/hour so as to be sequentiallysolidified from a lower portion thereof. In addition, when the sampleentered a region in which cooling water of the furnace was circulated,the difference in temperature at a solid-liquid interface which was theboundary between a portion at which the sample was melted and a portionat which the sample was solidified was set to 30° C./mm or more. Thesamples thus formed were each processed by cutting along a plane (thefirst principal surface or the second principal surface) perpendicularto the pull-down direction of the sample, and the transmission image wasobtained by an optical microscope. The results are shown in FIGS. 15A to15F.

FIG. 15A shows an image of the RbI—NaF system. From FIG. 15A, it wasfound that the structure was formed such that columnar crystals of NaFwere surrounded by RbI. This structure corresponded to the firstconfiguration, the first crystal phase was NaF, and the second crystalphase was RbI.

FIG. 15B shows an image of the RbI—NaCl system. From FIG. 15B, it wasfound that the structure was formed such that columnar crystals of NaClwere surrounded by RbI. This structure corresponded to the firstconfiguration, the first crystal phase was NaCl, and the second crystalphase was RbI.

FIG. 15C shows an image of the RbI—NaBr system. From FIG. 15C, it wasfound that the structure was formed such that plate crystals of NaBr andplate crystals of RbI were alternately disposed in close contact witheach other. This structure corresponded to the second configuration, thefirst crystal phase was RbI and the second crystal phase was NaBr.

FIG. 15D shows an image of the CsBr—NaF system. From FIG. 15D, it wasfound that the structure was formed such that columnar crystals of NaFwere surrounded by CsBr. This structure corresponded to the firstconfiguration, the first crystal phase was NaF, and the second crystalphase was CsBr.

FIG. 15E shows an image of the CsBr—NaCl system. From FIG. 15 (E), itwas found that the structure was formed such that columnar crystals ofNaCl were surrounded by CsBr. This structure corresponded to the firstconfiguration, the first crystal phase was NaCl, and the second crystalphase was CsBr.

FIG. 15F shows an image of the CsBr—NaBr system. From FIG. 15F, it wasfound that the structure was formed such that columnar crystals of NaBrwere surrounded by CsBr. This structure corresponded to the firstconfiguration, the first crystal phase was NaBr, and the second crystalphase was CsBr.

FIG. 16G shows an image of the RbBr—NaF system. From FIG. 16G, it wasfound that the structure was formed such that columnar crystals of NaFwere surrounded by RbBr. This structure corresponded to the firstconfiguration, the first crystal phase was NaF, and the second crystalphase was RbBr.

FIG. 16H shows an image of the RbBr—NaCl system. From FIG. 16H, it wasfound that the structure was formed such that plate crystals of NaCl andplate crystals of RbBr were alternately disposed in close contact witheach other. This structure corresponded to the second configuration, thefirst crystal phase was RbBr, and the second crystal phase was NaCl.

FIG. 16I shows an image of the RbBr—NaBr system. From FIG. 16I, it wasfound that the structure was formed such that columnar crystals of NaBrwere surrounded by RbBr. This structure corresponded to the thirdconfiguration, the first crystal phase was RbBr, and the second crystalphase was NaBr.

FIG. 16J shows an image of the CsCl—NaCl system. From FIG. 16J, it wasfound that the structure was formed such that columnar crystals of NaClwere surrounded by CsCl. This structure corresponded to the firstconfiguration, the first crystal phase was NaCl, and the second crystalphase was CsCl.

FIG. 16K shows an image of the RbCl—NaCl system. From FIG. 16K, it wasfound that the structure was formed such that columnar crystals of NaClwere surrounded by RbCl. This structure corresponded to the thirdconfiguration, the first crystal phase was RbCl, and the second crystalphase was NaCl.

FIG. 16L shows an image of the RbCl—NaF system. From FIG. 16L, it wasfound that the structure was formed such that columnar crystals of NaFwere surrounded by RbCl. This structure corresponded to the firstconfiguration, the first crystal phase was NaF, and the second crystalphase was RbCl.

Since these images were obtained by cutting the sample along the plane(the first principal surface, the second principal surface)perpendicular to the pull-down direction, the columnar crystal wasobserved as a point or a plate. However, it was confirmed that thecolumnar crystal was continuously formed in one direction in a planeparallel to the pull-down direction, and it could be confirmed that thephase separation structure of the present invention was formed by theunidirectional solidification. In this example, since scratches and/ordistortions of the image in some regions were caused, for example, bymarks formed by polishing in the sample formation, and blurs were causedby deliquescence of materials and deviation of focus due to an inclinedsample, the essence of the present invention was not influenced thereby.

In addition, the optical microscope images shown in FIGS. 15A to 16Lwere each obtained by a transmission arrangement, and it was confirmedthat the crystal phase which looked bright in each image was formed ofthe material at a higher refractive index side (second crystal phase)shown in Table 10; hence, it was shown that the scintillator crystalbody of the present invention had a light guide function by thedifference in refractive index.

According to those described above, it was confirmed that even if theprimary component of the second crystal phase which was a highrefractive-index crystal phase was formed of a material other than ascintillator material, a phase separation scintillator structure inwhich one of the two phases contained RbI, CsBr, RbBr, CsCl, or RbCl asa primary component also had a light guide property.

Example 13

This example relates to a concrete example in which another material wasadded to RbI, CsBr, or RbBr which formed one of the crystal phases.

As in the case of Example 12, the following material systems werevacuum-sealed in respective quartz tubes, and samples were formed.

As a concrete example in which CsI or RbBr was added to RbI which was aprimary component, (RbI85-CsI15)—NaCl and (RbI80-RbBr20)-NaCl wereformed. In addition, as a concrete example in which CsI, RbBr or CsClwas added to CsBr which was a primary component, (CsBr80-CsI20)-NaCl,(CsBr50-CsI50)-NaCl, (CsBr80-RbBr20)-NaCl, (CsBr80-CsCl20)-NaCl, and(CsBr60-CsCl40)-NaCl were formed. Furthermore, as a concrete example inwhich RbI or CsBr was added to RbBr which was a primary component,(RbBr95-RbI5)-NaCl, (RbBr50-RbI50)-NaCl, and (RbBr90-CsBr10)-NaCl wereformed. In the case of (RbI85-CsI15)-NaCl, the above compositionnotation indicates that the composition ratio of RbI to CsI is 85:15percent by mole. In addition, when materials forming one crystal phasewere mix-crystallized, it was very complicated to calculate a eutecticcomposition with a material forming the other crystal phase for eachcomposition; hence, NaCl was selected as a representative example of theprimary component of the other crystal phase, and a starting compositionwas formed in which 35 percent by mole of NaCl was contained withrespect to the one crystal phase. However, the formation was performedusing the formation apparatus shown in FIG. 2A so that a targetedstructure was not adversely influenced by a component deviated from thecomposition.

The sample thus formed was processed by cutting along a plane (the firstprincipal surface, the second principal surface) perpendicular to thepull-down direction as in the case of Example 12, and the transmissionimage was obtained by an optical microscope. The results are shown inFIGS. 17A to 17F.

FIG. 17A shows an image of the (RbI85-CsI15)-NaCl system. From FIG. 17A,it was found that the structure was formed such that NaCl columnarcrystals were surrounded by a (RbI85-CsI15) mixed crystal. Thisstructure corresponded to the first configuration, the first crystalphase was NaCl, and the second crystal phase was (RbI85-CsI15).

FIG. 17B shows an image of the (RbI80-RbBr20)-NaCl system. From FIG.17B, it was found that the structure was formed such that NaCl columnarcrystals were surrounded by a (RbI80-RbBr20) mixed crystal. Thisstructure corresponded to the first configuration, the first crystalphase was NaCl, and the second crystal phase was (RbI80-RbBr20).

FIG. 17C shows an image of the (CsBr80-CsI20)-NaCl system. From FIG.17C, it was found that the structure was formed such that NaCl columnarcrystals were surrounded by a (CsBr80-CsI20) mixed crystal. Thisstructure corresponded to the first configuration, the first crystalphase was NaCl, and the second crystal phase was (CsBr80-CsI20).

FIG. 17D shows an image of the (CsBr50-CsI50)-NaCl system. From FIG.17D, it was found that the structure was formed such that NaCl columnarcrystals were surrounded by a (CsBr50-CsI50) mixed crystal. Thisstructure corresponded to the first configuration, the first crystalphase was NaCl, and the second crystal phase was (CsBr50-CsI50).

FIG. 17E shows an image of the (CsBr80-RbBr20)-NaCl system. From FIG.17E, it was found that the structure was formed such that NaCl columnarcrystals were surrounded by a (CsBr80-RbBr20) mixed crystal. Thisstructure corresponded to the first configuration, the first crystalphase was NaCl, and the second crystal phase was (CsBr80-RbBr20). Inthis sample, a region in which NaCl columnar crystals were not straightwas generated by the influence in the sample formation, and dark regionslarger than intrinsic dark points to be observed were scattered in atransmission image; however, the essence of the present invention wasnot influenced thereby.

FIG. 17F shows an image of the (CsBr80-CsCl20)-NaCl system. From FIG.17F, it was found that the structure was formed such that NaCl columnarcrystals were surrounded by a (CsBr80-CsCl20) mixed crystal. Thisstructure corresponded to the first configuration, the first crystalphase was NaCl, and the second crystal phase was (CsBr80-CsCl20).

FIG. 18G shows an image of the (CsBr60-CsCl40)-NaCl system. From FIG.18G, it was found that the structure was formed such that NaCl columnarcrystals were surrounded by a (CsBr60-CsCl40) mixed crystal. Thisstructure corresponded to the first configuration, the first crystalphase was NaCl, and the second crystal phase was (CsBr60-CsCl40).

FIG. 18H shows an image of the (RbBr95-RbI5)-NaCl system. From FIG. 18H,it was found that the structure was formed such that NaCl plate crystalsand plate crystals of a (RbBr95-RbI5) mixed crystal were alternatelydisposed in close contact with each other. This structure correspondedto the second configuration, the first crystal phase was (RbBr95-RbI5),and the second crystal phase was NaCl.

FIG. 18I shows an image of the (RbBr50-RbI50)-NaCl system. From FIG.18I, it was found that the structure was formed such that NaCl columnarcrystals were surrounded by a (RbBr50-RbI50) mixed crystal. Thisstructure corresponded to the first configuration, the first crystalphase was NaCl, and the second crystal phase was (RbBr50-RbI50).

FIG. 18J shows an image of the (RbBr90-CsBr10)-NaCl system. From FIG.18J, it was found that the structure was formed such that NaCl platecrystals and plate crystals of a (RbBr90-CsBr10) mixed crystal werealternately disposed in close contact with each other. This structurecorresponded to the second configuration, the first crystal phase was(RbBr90-CsBr10), and the second crystal phase was NaCl. In addition, theoptical microscope images shown in FIGS. 17A to 18J were each obtainedby a transmission arrangement, and it was confirmed that the crystalphase which looked bright in each image was formed of the material at ahigher refractive index side (second crystal phase); hence, it was shownthat the scintillator crystal body of the present invention had a lightguide function by the difference in refractive index.

As described above, it was confirmed that even if the mixed crystal wasused for one crystal phase of the present invention instead of using asingle material, this structure functioned as a phase separationscintillator and also had a light guide property.

Example 14

This example relates to the addition of a luminescence center.

By using a formation method similar to that of Example 12, samples wereformed by combinations in which 0.01 percent by mole of TlI (thalliumiodide), InI (indium iodide), and Ga (gallium) were separately added toall material systems. When these samples were excited by x rays obtainedat 60 kV and 1 mA using a tungsten bulb, in all the combinations, theluminescence of the crystal body could be confirmed by visualinspection.

Hence, it was confirmed that the phase separation structure of thepresent invention functioned as a scintillator crystal body which couldemit light by radiation excitation.

Example 15

This example relates to the composition of a phase separationscintillator.

From three types of samples of CsBr—NaCl (30, 38, and 40 percent bymole) shown by arrows in an equilibrium diagram of a CsBr—NaCl system ofFIG. 19, three types of phase separation scintillators were formed usingan apparatus similar to that of Example 1 shown in FIG. 2A. In addition,from these three types of samples, three types of phase separationscintillators were formed using the apparatus shown in FIG. 2B in whichthe heater portion was narrow and the sample was locally heated. Asdescribed above, totally six types of phase separation scintillatorswere prepared. In this example, CsBr—NaCl (30 percent by mole) indicatesthat the molar ratio of CsBr to NaCl is 70:30. In the same manner asdescribed above, CsBr—NaCl (38 percent by mole) and CsBr—NaCl (40percent by mole) also indicate that the molar ratios of CsBr to NaCl are62:38 and 60:40, respectively.

In the case in which solidification was started after the entire samplewas melted at an early stage as in the case of Example 12, from all thesamples, scintillators each having an excellent structure as shown inFIG. 15E were obtained. However, in the case of CsBr—NaCl (30 percent bymole), a crystal was formed together with precipitation of CsBrdendrites in a solidification initial area of the sample, andthereafter, a region of an excellent unidirectional phase separationstructure was formed. As for CsBr—NaCl (38 percent by mole) andCsBr—NaCl (40 percent by mole), a dendrite region as that of CsBr—NaCl(30 percent by mole) was not clearly formed at an early stage of thesolidification, and a significant difference was not seen between theabove two samples. However, since the difference caused by thedifference in concentration of 2 percent by mole may also be partiallyinfluenced by impurities of raw materials, it did not indicate that nodifference was present. As described above, when the entire sample wasmelted, since a material deviated from a eutectic composition wasprecipitated in the solidification initial area of the sample at ahigher temperature than the eutectic point. Hence, it is believed that aremaining melt portion was converged to the eutectic composition, andsubsequently, preferable structure formation was performed.

In addition, when solidification was performed while a local area of thesample was being melted as shown in FIG. 2B, CsBr dendrites were presentat any place of the sample of the CsBr—NaCl (30 percent by mole) system,and a targeted structure was not obtained. However, in the case ofCsBr—NaCl (38 percent by mole) and CsBr—NaCl (40 percent by mole), thecolumnar crystals of CsBr were formed over the entire area and anexcellent sample could be obtained. However, it was observed thatalthough being influenced by impurities and the like, the structure ofthe sample of CsBr—NaCl (38 percent by mole) was more distorted thanthat of the sample of CsBr—NaCl (40 percent by mole). Hence, when thesolidification was performed by local melting, it was found that sincethis solidification was not subjected to a process in which thecomposition was optimized in the sample when the entire was melted, thecomposition and the structure sensitively influenced each other.

Hence, in the phase separation scintillator crystal of the presentinvention, although a slight fluctuation of the composition, such asapproximately 2 percent by mole, had not considerable adverse influenceon the structure formation, since a considerable fluctuation of 10percent by mole had influences, it was clear that the optimalcomposition was in the vicinity of the eutectic composition. Inaddition, it was found that when solidification was controlled after theentire sample was melted in the manufacturing, even if the compositionwas deviated from the eutectic composition, a deviated material waspreferentially precipitated in the solidification initial area of thesample, and an excellent region could be obtained from the remainingmelt of the eutectic composition, and this finding is important.

Example 16

Examples 16 and 17 are examples corresponding to the above fourthembodiment and will be described in this order.

In this example, first, powders of five material systems were preparedin which BaI₂—NaI, BaBr₂—NaBr, BaCl₂—NaCl, SrBr₂—NaBr, and SrCl₂—NaCleach had mixing ratios of 50:50, 40:60, 40:60, 60:40, and 52:48, andwere separately vacuum-sealed in respective quartz tubes, so thatsamples were formed. Next, after the sample was placed in a Bridgmanfurnace as shown by the perspective view of FIG. 2A and was increased intemperature to 800° C. so that the entire sample was sufficientlymelted, the temperature was held for 30 minutes, and the melttemperature was then decreased to a temperature 20° C. higher than theeutectic point shown in Table 12. Subsequently, the sample was pulleddown at a rate of approximately 10 mm/hour so that the sample wassequentially solidified from a lower portion thereof.

In addition, when the sample entered a region in which cooling water ofthe furnace was circulated by the pull-down of the sample, thedifference in temperature at a solid-liquid interface which was theboundary between a portion at which the sample was melted and a portionat which the sample was solidified was set to 30° C./mm or more. Fivetypes of samples thus formed were each processed by cutting, and thetransmission image (image of the first principal surface, the secondprincipal surface) was obtained by an optical microscope. As a result,images of FIGS. 20A to 20E of the five material systems were obtained.Since the phases of BaI₂, BaBr₂, BaCl₂, SrBr₂, and SrCl₂, each having ahigh refractive index, looked bright, and the low refractive-index phaselooked dark, it was confirmed that a desired structure of the presentinvention could be formed, and at the same time, a light guide functionwas obtained.

In this example, since scratches and/or distortions of the image in someregions were caused by marks formed by polishing in the sampleformation, and blurs were caused by deliquescence of materials anddeviation of focus due to an inclined sample, the essence of the presentinvention was not influenced thereby.

Example 17

This example relates to the addition of a luminescence center.

By a formation method similar to that of Example 16, samples were formedin which five material systems, that is, BaI₂—NaI, BaBr₂—NaBr,BaCl₂—NaCl, SrBr₂—NaBr, and SrCl₂—NaCl, were each added with 0.5 percentby mole of one selected from EuI₂, EuBr₂, and EuCl₂ so that the halogenscoincided therebetween. When the luminescence was confirmed by x-rayexcitation, spectra having peaks at approximately 420 nm, 400 nm, 400nm, 400 nm, and 410 nm were obtained, and it was found that the responseto radiation could be obtained by addition of the luminescence center.

In addition, as in the case of Example 8 described above, when theradiation detector shown in FIG. 21 was formed such that the phaseseparation scintillator crystal body described in one of Examples 9 to17 described above was disposed on a photodetector array so that thefirst principal surface or the second principal surface facesphotodetectors, it was also confirmed from an output of thephotodetector array that compared to a radiation detector using a singlecrystal scintillator without using partitions, the spread of light wassuppressed.

Since emitting light by radiation and having properties of guidingemitted light, when the phase separation scintillator crystal body ofthe present invention is used in combination with a photodetectorwithout forming related partitions, this combination is quite useful asa radiation detector. In particular, the phase separation scintillatorcrystal body of this embodiment can be used, for example, as ameasurement apparatus for medical use, industrial use, high-energyphysical use, and space use, in each of which radiation, such as x rays,is used.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2010-017378, filed Jan. 28, 2010, No. 2010-088982, filed Apr. 7, 2010,No. 2010-162017, filed Jul. 16, 2010, and No. 2010-269728, filed Dec. 2,2010, which are hereby incorporated by reference herein in theirentirety.

The invention claimed is:
 1. A scintillator having a first surface and asecond surface that are not located on the same plane, the scintillatorcomprising: a first crystal phase and a second crystal phase, whereinthe first crystal phase forms, together with the second crystal phase, aeutectic structure, and has a refractive index that allows totalreflection of light from the second crystal phase, each of the firstcrystal phase and the second crystal phase continuously extends from asurface portion of the first surface to a surface portion of the secondsurface, and the first crystal phase is located around the secondcrystal phase, or the second crystal phase is located around the firstcrystal phase.
 2. The scintillator crystal body according to claim 1,wherein the first crystal phase or the second crystal phase has a shapeof a column or a plate.
 3. The scintillator according to claim 2,comprising a plurality of the first crystal phases having a shape of acolumn or a plate or a plurality of the second crystal phases having ashape of a column or a plate, and the plurality of crystal phases areperiodically arranged along the first surface or the second surface at aperiod of 500 nm or more and 50 μm or less.
 4. The scintillatoraccording to claim 1, wherein the second crystal phase emits light byradiation excitation.
 5. The scintillator according to claim 1, whereina composition ratio of the first crystal phase and the second crystalphase is within ±4 percent by mole with respect to a eutecticcomposition ration of the first crystal phase and the second crystalphase.
 6. The scintillator according to claim 1, wherein the firstcrystal phase or the second crystal phase includes at least one of Tl,In, and Ga as a luminescence center.
 7. The scintillator crystal bodyaccording to claim 1, wherein the first crystal phase or the secondcrystal phase contains CsI.
 8. The scintillator according to claim 7,wherein the first crystal phase or the second crystal phase thatcontains CSI contains one of RbI, CsBr, and RbBr, and the content of RbIis in a range of more than 0 to 20 percent by mole, the content of CsBris in a range of more than 0 to less than 50 percent by mole, and thecontent of RbBr is in a range of more than 0 to 10 percent by mole. 9.The scintillator according to claim 1, wherein the second crystal phasecontains CsI, and the first crystal phase contains one of NaBr, NaCl,NaF, and KCl.
 10. The scintillator crystal body according to claim 1,wherein the first crystal phase or the second crystal phase containsNaI.
 11. The scintillator according to claim 1, wherein the secondcrystal phase contains NaI, and a phase of the first crystal phasecontains one of NaF, NaCl, RbI and CsI.
 12. The scintillator accordingto claim 1, wherein the first crystal phase or the second crystal phasecontains RbI.
 13. The scintillator according to claim 1, wherein thefirst crystal phase or the second crystal phase contains CsBr.
 14. Thescintillator according to claim 1, wherein the first crystal phase orthe second crystal phase contains RbBr.
 15. The scintillator accordingto claim 1, wherein one phase of the first crystal phase and the secondcrystal phase contains one of RbI, CsBr, and RbBr, and the other phaseof the crystal phases contains one of NaF, NaCl, and NaBr.
 16. Thescintillator according to claim 15, wherein one crystal phase of thefirst crystal phase and the second crystal phase contains RbI, the onecrystal phase contains CsI or RbBr, the content of CsI is in a range ofmore than 0 to 20 percent by mole, and the content of RbBr is in a rangeof more than 0 to less than 50 percent by mole.
 17. The scintillatoraccording to claim 15, wherein one crystal phase of the first crystalphase and the second crystal phase contains CsBr, the one crystal phasecontains one of CsI, RbBr, and CsCl, the content of CsI is in a range ofmore than 0 to 50 percent by mole, the content of RbBr is in a range ofmore than 0 to less than 25 percent by mole, and the content of CsCl isin a range of more than 0 to less than 50 percent by mole.
 18. Thescintillator according to claim 15, wherein one crystal phase of thefirst crystal phase and the second crystal phase contains RbBr, the onecrystal phase contains RbI or CsBr, the content of RbI is in a range ofmore than 0 to 50 percent by mole, and the content of CsBr is in rangeof more than 0 to 15 percent by mole.
 19. The scintillator according toclaim 1, wherein the second crystal phase contains one of BaI₂, BaBr₂,BaCl₂, SrBr₂, and SrCl₂.
 20. The scintillator according to claim 19,wherein one of the first crystal phase and the second crystal phaseincludes at least one of europium (Eu), cerium (Ce), thallium (Tl),indium (In), and gallium (Ga) as a luminescence center.
 21. A radiationdetector comprising a photodetector and a scintillator disposed to facethe photodetector, wherein the scintillator is the scintillatoraccording to claim
 1. 22. The radiation detector according to claim 21,wherein the photodetector and the scintillator are disposed to face eachother with or without at least one protective layer interposedtherebetween.
 23. A scintillator having a first surface and a secondsurface that are not located on the same plane, the scintillatorcomprising a first crystal phase and a second crystal phase, wherein thefirst crystal phase forms, together with the second crystal phase, aeutectic structure, and has a refractive index that is 0.973 or lesstimes a refractive index of the second crystal phase, the second crystalphase emits light by radiation excitation, each of the first crystalphase and the second crystal phase continuously extends from a surfaceportion of the first surface to a surface portion of the second surface,and the first crystal phase is located around the second crystal phase,or the second crystal phase is located around the first crystal phase.24. A radiation detector comprising a photodetector and a scintillatordisposed to face the photodetector, wherein the scintillator is thescintillator according to claim
 23. 25. A scintillator having a firstsurface and a second surface that are not located on the same plane, thescintillator comprising a first crystal phase and a second crystalphase, wherein the first crystal phase forms, together with the secondcrystal phase, a eutectic structure, emits light by radiationexcitation, and has a refractive index that is 0.987 or less times arefractive index of the second crystal phase, each of the first crystalphase and the second crystal phase continuously extends from a surfaceportion of the first surface to a surface portion of the second surface,and the first crystal phase is located around the second crystal phase,or the second crystal phase is located around the first crystal phase.26. A radiation detector comprising a photodetector and a scintillatordisposed to face the photodetector, wherein the scintillator is thescintillator according to claim 25.